security: upgrade high-severity vulnerable transitive dependencies and enable Dependabot alerts

[teesofttech]

Why

dotnet list PayBridge.SDK.sln package --vulnerable --include-transitive reports high-severity advisories in SQLitePCLRaw.lib.e_sqlite3 2.1.6 and System.Text.Json 8.0.0 across the SDK, tests, example, and presentation projects. Repository vulnerability alerts are currently disabled.

Scope

• Upgrade the .NET 8 dependency graph to patched, compatible versions.
• Prefer current .NET 8 servicing releases rather than moving target frameworks as part of this fix.
• Enable Dependabot vulnerability alerts and dependency update configuration.
• Add a CI dependency-vulnerability check with an explicit severity policy.

Acceptance criteria

• The vulnerable-package command reports no known high/critical vulnerabilities.
• Build and all unit tests pass.
• Dependabot alerts are enabled.
• A scheduled dependency check fails on new high/critical advisories.
• Upgrade notes identify any behavior or migration risk.