Fix version metadata loading fallback and bounds

[Widthdom]

Summary

• Make ConsoleUi.LoadVersion fail softly when version.json is missing, unreadable, malformed, or schema-invalid.
• Bound version.json reads to 16 KiB and parse with an explicit JSON depth limit.
• Add focused ConsoleUi regression coverage for malformed, missing, oversized, too-deep, and non-string version metadata.

Validation

• dotnet build
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~ConsoleUiTests.LoadVersionFromFile|FullyQualifiedName~ConsoleUiTests.LoadVersion_ReturnsActualVersion"
• 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 exec --ignore-user-config review --base origin/main --ephemeral

Documentation / Changelog

• Added changelog.d/unreleased/3025.fixed.md.
• Added changelog.d/unreleased/3244.fixed.md.
• No README or guide update was needed because the change only hardens internal version metadata loading behavior.

Follow-up Candidates

• None.

Fixes #3025
Fixes #3244