Use IMemoryCache for process-local tool embeddings

[KSemenenko]

Summary

• replace the built-in process-local embedding store internals with IMemoryCache
• add AddMcpGatewayInMemoryToolEmbeddingStore() and update README, architecture docs, and ADRs
• remove obsolete/legacy leftovers and update tests around cache-backed behavior

Verification

• dotnet restore ManagedCode.MCPGateway.slnx
• dotnet build ManagedCode.MCPGateway.slnx -c Release --no-restore
• dotnet build ManagedCode.MCPGateway.slnx -c Release --no-restore -p:RunAnalyzers=true
• dotnet test --solution ManagedCode.MCPGateway.slnx -c Release --no-build
• roslynator analyze src/ManagedCode.MCPGateway/ManagedCode.MCPGateway.csproj -p Configuration=Release --severity-level warning
• roslynator analyze tests/ManagedCode.MCPGateway.Tests/ManagedCode.MCPGateway.Tests.csproj -p Configuration=Release --severity-level warning
• cloc --include-lang=C# src tests

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[Copilot]

Pull request overview

This PR updates the gateway’s process-local tool embedding reuse to be backed by IMemoryCache, adds a dedicated DI registration helper for that store, and cleans up legacy/obsolete paths and documentation to match the new design.

Changes:

• Reimplemented McpGatewayInMemoryToolEmbeddingStore on top of IMemoryCache (including deterministic fingerprint-agnostic fallback behavior) and removed the old internal index helper.
• Added AddMcpGatewayInMemoryToolEmbeddingStore() for DI registration and updated tests/docs/ADRs accordingly.
• Renamed “legacy” lexical naming to “heuristic” in runtime search internals and refreshed related test data.

Reviewed changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/ManagedCode.MCPGateway.Tests/TestSupport/TestToolEmbeddingStore.cs	Updates test embedding store to track embeddings directly after internal index removal.
tests/ManagedCode.MCPGateway.Tests/Search/McpGatewaySearchLexicalTests.cs	Renames a test context key from legacy to compatibility.
tests/ManagedCode.MCPGateway.Tests/Search/McpGatewayInitializationTests.cs	Adds DI test coverage for AddMcpGatewayInMemoryToolEmbeddingStore() and shared IMemoryCache.
tests/ManagedCode.MCPGateway.Tests/Search/McpGatewayInMemoryToolEmbeddingStoreTests.cs	Updates disposal pattern and adds test for “latest embedding wins” fallback behavior.
src/ManagedCode.MCPGateway/Registration/McpGatewayServiceCollectionExtensions.cs	Adds AddMcpGatewayInMemoryToolEmbeddingStore() (registers IMemoryCache + store).
src/ManagedCode.MCPGateway/Registration/McpGatewayChatOptionsExtensions.cs	Removes obsolete AddManagedCodeMcpGatewayTools(...) shims.
src/ManagedCode.MCPGateway/ManagedCode.MCPGateway.csproj	Adds package reference for Microsoft.Extensions.Caching.Memory.
src/ManagedCode.MCPGateway/Internal/Runtime/Search/McpGatewayRuntime.TokenSearch.cs	Renames legacy lexical scoring concepts to heuristic lexical scoring.
src/ManagedCode.MCPGateway/Internal/Runtime/Core/McpGatewayRuntime.cs	Renames lexical feature weight constant to heuristic naming.
src/ManagedCode.MCPGateway/Internal/Runtime/Core/McpGatewayRuntime.Types.cs	Renames retrieval candidate score field to heuristic naming.
src/ManagedCode.MCPGateway/Internal/Embeddings/McpGatewayToolEmbeddingStoreIndex.cs	Removes the old internal embedding index helper implementation.
src/ManagedCode.MCPGateway/Embeddings/McpGatewayInMemoryToolEmbeddingStore.cs	Implements the store using IMemoryCache and supports DI-owned vs self-owned cache lifetimes.
docs/Architecture/Overview.md	Updates architecture diagrams/text to reflect IMemoryCache backing and module list cleanup.
docs/ADR/ADR-0004-process-local-embedding-store-uses-imemorycache.md	Adds ADR documenting the IMemoryCache decision and invariants.
README.md	Updates embedding-store guidance to use AddMcpGatewayInMemoryToolEmbeddingStore() and clarifies process-local scope.
Directory.Packages.props	Adds package version for Microsoft.Extensions.Caching.Memory.
AGENTS.md	Adds explicit repo guidance favoring framework caching primitives and removing legacy leftovers.

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[KSemenenko]

Addressed the memory-cache review feedback in f35237a.

Changes:

• set an explicit cache-entry Size for both exact and fallback IMemoryCache writes so the store works with shared caches configured with MemoryCacheOptions.SizeLimit
• added regression coverage for McpGatewayInMemoryToolEmbeddingStore against a size-limited shared MemoryCache

Verified locally:

• dotnet restore ManagedCode.MCPGateway.slnx
• dotnet build ManagedCode.MCPGateway.slnx -c Release --no-restore
• dotnet build ManagedCode.MCPGateway.slnx -c Release --no-restore -p:RunAnalyzers=true
• dotnet test --solution ManagedCode.MCPGateway.slnx -c Release --no-build
• roslynator analyze for src and tests
• cloc --include-lang=C# src tests