Fix nested read migration transactions

[Widthdom]

Summary

• Avoid starting a nested SQLite transaction when TryMigrateForRead is invoked inside an already-active transaction.
• Preserve the existing degraded migration behavior for read-only/open failures.
• Add regression coverage for the existing-transaction path.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~DatabaseTests.TryMigrateForRead_InsideExistingTransaction_DoesNotStartNestedTransaction|FullyQualifiedName~ConcurrencyTests.GetRepoMap_FreshnessAndEntrypointsStaySnapshotConsistent_UnderConcurrentWriter|FullyQualifiedName~LegacySchemaMigrationTests.TryMigrateForRead_PartialDdlFailure_RecordsStepAndEmitsActionableWarning" -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• git diff --check origin/main..HEAD
• dotnet test CodeIndex.sln -p:UseSharedCompilation=false on latest merged origin/main: net8 passed; net9 failed in QueryCommandRunnerTests.RunPublishedTrimmedCli_SearchSupportsCSharpRazorAliases(lang: "cshtml") due the known macOS arm64 ILLink AccessViolation tracked by macOS arm64 trimmed publish test can crash in ILLink #2606.
• Adversarial review: No blocking/actionable issues found.

Documentation and Changelog

• Added changelog.d/unreleased/2607.fixed.md.
• No docs changes were needed; the fix preserves the existing read-migration contract.

Follow-up Candidates

• None.

Fixes #2607