Fix read migration writer lock race

[Widthdom]

Summary

• Skip the read-path legacy schema migration when the current database already has the required tables, columns, and indexes.
• Avoid taking a SQLite writer lock for no-op read migration checks on up-to-date databases.
• Add a regression test for calling TryMigrateForRead while another connection holds an active write transaction.

Validation

• dotnet build CodeIndex.sln -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-build --no-restore --filter FullyQualifiedName~LegacySchemaMigrationTests
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-build --no-restore --filter FullyQualifiedName~ConcurrencyTests
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --no-build --no-restore --filter FullyQualifiedName~LegacySchemaMigrationTests
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --no-build --no-restore --filter FullyQualifiedName~ConcurrencyTests
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet format CodeIndex.sln --verify-no-changes
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review: no blocking/actionable issues found

Documentation / Changelog

• Added changelog.d/unreleased/2932.fixed.md.

Follow-Up Candidates

• None.

Fixes #2932