Fix SQLite URI connection string handling

[Widthdom]

Summary

• Route user-supplied SQLite database paths through SqliteConnectionStringBuilder for CLI db, diff, metadata reads, and DbContext read-only URI opens.
• Keep ;Mode=...-style payloads inside the data source value instead of allowing them to become connection-string options.
• Add regression coverage for semicolon payload parsing and missing-database creation prevention.

Validation

• dotnet build
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~DbPathResolverTests|FullyQualifiedName~DbCommandRunnerTests"
• dotnet test (net8 passed; net9 had one unrelated PostExtractionHookTests.CallbackBudgetExceeded_AddsDiagnosticAndSkipsTimedOutMutation failure, and the same test passed on targeted rerun)
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~PostExtractionHookTests.CallbackBudgetExceeded_AddsDiagnosticAndSkipsTimedOutMutation"
• 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

Documentation / Changelog

• Added changelog.d/unreleased/3220.security.md.
• No documentation update beyond the changelog fragment: this is a security hardening of existing SQLite URI handling, not a CLI contract change.

Review

Adversarial review: No blocking/actionable issues found.

Follow-up Candidates

None.

Fixes #3220