Fix Python dataclass field extraction

[Widthdom]

Summary

• Distinguish Python dataclass field(...) class attributes and index field metadata keys.
• Emit references for dataclass field default factories, metadata keys, and fields(MyClass) introspection.
• Add focused coverage for multiline dataclass field(...) calls and multiline metadata dictionaries.

Fixes #2057

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter Dataclass --no-restore -p:UseSharedCompilation=false
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false

Documentation / changelog

• Added changelog fragment: changelog.d/unreleased/2057.fixed.md

Review

• Adversarial review round 1 found multiline field references were missing; fixed.
• Adversarial review round 2 found multiline metadata dict references were missing; fixed.

Follow-up candidates

• cdidx index can stall indefinitely on large test files #2627