Preserve Python reference columns after string literals

[Widthdom]

Summary

Python string masking replaced literals with "", shifting subsequent reference columns. Preserve their UTF-16 width so s = '😀'; 終了() records column 15 and LSP reports character 14. Repeated calls retain separate source positions, and string/comment decoys remain excluded.

Bump the Python extractor contract to 3 so a normal whole-workspace index refresh repairs unchanged files written under contracts 1 or 2. No forced rebuild or schema migration is required.

Fixes #5362

Validation

• Reproduced the pre-fix column regression on .NET 8 and .NET 9.
• Verified real UTF-8 indexing, persisted resolved reference rows, CLI JSON columns and initialized Content-Length-framed LSP ranges across empty, ASCII, BMP, astral, escaped, raw, byte, adjacent, triple-quoted and literal f-strings.
• Verified normal refresh from Python extractor contracts 1 and 2, retaining unrelated-file reuse.
• Symbol/reference extractor, LSP and focused indexing regressions: .NET 8 passed 2,677 tests; .NET 9 passed 2,648 tests with 29 intended runtime-specific skips.
• Post-commit solution build passed with zero warnings/errors (--no-restore -m:1 -nr:false -p:UseSharedCompilation=false).
• Repository-built CLI smoke: lib.py:4:15, resolved target, one candidate.
• Changelog fragment validation and git diff --check passed.
• The CI whitespace check passed locally using dotnet format whitespace CodeIndex.sln --verify-no-changes --no-restore --verbosity minimal (the command behind dev.sh lint).
• Post-commit dogfood refresh passed; normal full indexing refreshed the remaining Python contracts while skipping 1,576 unchanged files. Root freshness, folded-name readiness, index completeness and graph completeness are healthy.

Documentation

• Added bilingual coordinate and refresh guidance in docs/python-reference-coordinates.md, linked from both developer-guide sections.
• Updated both testing-guide sections.
• Changelog fragment: changelog.d/unreleased/5362.fixed.md.

Adversarial review

Codex reviewed origin/main..HEAD and the subsequent whitespace-only CI fix in two rounds: No blocking/actionable issues found.