Add SQL FK regression coverage for #1443

[Widthdom]

Summary

• Adds regression coverage for SQL ALTER TABLE ... ADD CONSTRAINT ... FOREIGN KEY ... REFERENCES ... extraction.
• Verifies inline and ALTER-style foreign keys both emit references to the referenced table.
• Adds a bilingual changelog fragment for SQL: ALTER TABLE ADD CONSTRAINT FK does not emit reference edge #1443.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~ReferenceExtractorTests"
• dotnet run --project tools/CodeIndex.Changelog -- check
• Codex adversarial review: No blocking/actionable issues found.

Documentation / Changelog

• Added changelog.d/unreleased/1443.fixed.md.
• No README or developer-guide updates were needed because this only locks existing SQL reference extraction behavior with regression coverage.

Follow-up candidates

• None.

Fixes #1443