build: adopt Central Package Management + lockfiles + transitive audit

[TalZaccai]

Summary

Modernizes the .NET package management stack across the whole repo as preparation for an upcoming fix-dependabot-alerts workflow port (microsoft/TypeAgent / microsoft/TypeChat equivalent). With CPM + transitive pinning in place, automated transitive vulnerability remediation becomes a one-line edit to Directory.Packages.props per fix instead of editing many csproj files.

Pairs naturally with #319 (Dependabot grouping config) but can merge independently.

What changes

Feature	What it does	File
Central Package Management	Single source of truth for every package version	new Directory.Packages.props
Central Transitive Pinning	<PackageVersion> forces a transitive across all projects	same
NuGetAudit transitive mode	NU190x warnings for both direct and transitive vulns at restore time	new Directory.Build.props

All 20 SDK-style csproj files become version-less <PackageReference> entries. Per-project version sprawl collapses (e.g. Microsoft.NET.Test.Sdk previously had 2 versions across the repo).

Vulnerabilities discovered and fixed by the new audit

The transitive-mode audit immediately surfaced two existing advisories that Dependabot had been blind to (transitive scope wasn't being audited):

Package	Was	Now	Advisory
Microsoft.SemanticKernel	1.68.0	1.71.0	GHSA-2ww3-72rp-wpp4 — critical: arbitrary file write via AI agent function calling
Microsoft.Bcl.Memory (transitive)	10.0.2	10.0.4 (pinned)	GHSA-73j8-2gch-69rq — high: DoS via Base64Url out-of-bounds read

Other version reconciliations

Conflicting versions across projects were collapsed to the highest. The orphan tests/TypeChat.Tests.Pre6 net470 project uses VersionOverride to keep its older xunit / Microsoft.NET.Test.Sdk pins (newer versions dropped net470):

• Microsoft.NET.Test.Sdk: 18.0.1 (was 17.5.0 + 18.0.1); Pre6 VersionOverride=17.5.0
• xunit: 2.9.3 (was 2.4.2 + 2.9.3); Pre6 VersionOverride=2.4.2
• xunit.runner.visualstudio: 3.1.5 (was 2.4.5 + 3.1.5); Pre6 VersionOverride=2.4.5

Other bumps required by the SK 1.71 transitive graph:

• System.Text.Json 10.0.0 → 10.0.2
• Microsoft.Extensions.AI 9.10.2 → 10.2.0
• System.Numerics.Tensors 10.0.1 → 10.0.2

On lockfiles (intentionally not adopted)

An earlier revision of this PR committed packages.lock.json files for every project. Per reviewer feedback that was reverted in commit b57d385:

• NuGet lockfiles are an opt-in feature; the .NET community (including dotnet/runtime) does not commit them by default.
• Without dotnet restore --locked-mode enforced in CI, committed lockfiles are write-only churn that adds noise to every dependency PR without providing determinism.
• Determinism / drift-detection can be added later as a focused follow-up PR (enable RestorePackagesWithLockFile + add --locked-mode to CI) without needing this scaffolding to land first.

packages.lock.json is now in .gitignore so opportunistic local restores with --use-lock-file don't get committed back.

Local validation

• dotnet restore TypeChat.sln: ✅ clean, zero NU190x audit warnings
• dotnet restore tests/EmojiApp/Emoji.sln: ✅ clean
• dotnet build TypeChat.sln -c Release /warnaserror (CI's exact build line): ✅ 0 warnings, 0 errors
• dotnet test tests/TypeChat.UnitTests: ✅ 154 passed, 0 failed

Notes for reviewers

• The two new top-level files (Directory.Build.props + Directory.Packages.props) carry the policy; each existing src/Directory.Build.props and examples/Directory.Build.props now explicitly <Import> the root so the MSBuild auto-discovery chain isn't truncated.
• tests/TypeChat.Tests.Pre6 (net470) is an orphan — not in TypeChat.sln and not built by CI. Pre-existing; unrelated to this PR.

Followup PR

A separate PR will add the fix-dependabot-alerts workflow (modeled on the microsoft/TypeAgent equivalent) that takes advantage of this scaffold.

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Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

[Copilot]

Pull request overview

Adopts .NET Central Package Management (CPM) repository-wide, introduces a new Directory.Build.props enabling transitive NuGet audit, and uses the resulting audit signal to bump two vulnerable packages (Microsoft.SemanticKernel → 1.71.0 and a pinned transitive Microsoft.Bcl.Memory 10.0.4). All 20 SDK-style csproj files become version-less <PackageReference> entries, with the legacy tests/TypeChat.Tests.Pre6 net470 project using VersionOverride to retain its older xunit/test SDK pins.

Changes:

• Add root Directory.Packages.props (CPM + transitive pinning) and Directory.Build.props (NuGetAudit=true, NuGetAuditMode=all); existing src/ and examples/ Directory.Build.props explicitly <Import> the root so MSBuild's first-hit walk-up isn't truncated.
• Strip Version= attributes from every <PackageReference> across src/, examples/, tests/, and src/package/nuget.props; reconcile previously-conflicting versions to the highest (e.g. Microsoft.NET.Test.Sdk 18.0.1, xunit 2.9.3) and apply VersionOverride in the net470 Pre6 test project.
• Bump versions surfaced by the new transitive audit: Microsoft.SemanticKernel 1.68.0 → 1.71.0, transitive Microsoft.Bcl.Memory pinned to 10.0.4, System.Text.Json 10.0.0 → 10.0.2, Microsoft.Extensions.AI 9.10.2 → 10.2.0, System.Numerics.Tensors 10.0.1 → 10.0.2.

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
Directory.Packages.props	New CPM manifest; single source of truth for all package versions, enables transitive pinning, pins vulnerable transitive Microsoft.Bcl.Memory 10.0.4.
Directory.Build.props	New repo-root props enabling NuGetAudit in all (transitive) mode.
src/Directory.Build.props	Explicitly imports root Directory.Build.props so MSBuild walk-up isn't stopped here.
examples/Directory.Build.props	Same explicit root import as src/.
src/package/nuget.props	Removes Version from Microsoft.SourceLink.GitHub reference.
src/typechat/TypeChat.csproj	Drops versions on System.ComponentModel.Annotations, System.Text.Json, Microsoft.CodeAnalysis.CSharp.
src/typechat.schema/TypeChat.Schema.csproj	Drops versions on Microsoft.CodeAnalysis.CSharp, System.ComponentModel.Annotations, System.Text.Json.
src/typechat.program/TypeChat.Program.csproj	Drops versions on Microsoft.CodeAnalysis.CSharp, Microsoft.CSharp.
src/typechat.sk/TypeChat.SemanticKernel.csproj	Drops versions on Azure.Identity, Microsoft.CSharp, Microsoft.Extensions.Configuration.Binder, Microsoft.SemanticKernel, System.Numerics.Tensors (effective SK bump to 1.71.0 via CPM).
src/typechat.meai/TypeChat.Extensions.AI.csproj	Drops versions on Azure.Identity, Microsoft.CSharp, Microsoft.Extensions.AI, Microsoft.Extensions.Configuration.Binder.
examples/typechat.examplesLib/TypeChat.ExamplesLib.csproj	Drops versions on Azure.Identity, Microsoft.Extensions.Configuration.*, System.CommandLine.
tests/EmojiApp/Emoji.csproj	Drops versions on the three published Microsoft.TypeChat* packages.
tests/TypeChat.TestLib/TypeChat.TestLib.csproj	Drops versions on Microsoft.CSharp, Microsoft.Extensions.Configuration.Json, Microsoft.NET.Test.Sdk, xunit, xunit.runner.visualstudio.
tests/TypeChat.UnitTests/TypeChat.UnitTests.csproj	Drops versions on Microsoft.NET.Test.Sdk, xunit, xunit.runner.visualstudio.
tests/TypeChat.IntegrationTests/TypeChat.IntegrationTests.csproj	Drops versions on Microsoft.NET.Test.Sdk, xunit, xunit.runner.visualstudio, Xunit.SkippableFact.
tests/TypeChat.Tests.Pre6/TypeChat.Tests.Pre6.csproj	Uses VersionOverride for net470-compatible older Microsoft.NET.Test.Sdk 17.5.0, xunit 2.4.2, xunit.runner.visualstudio 2.4.5; drops versions on System.Linq and coverlet.collector.

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