fix: add context-aware diagnostics for class-string<static>

[calebdw]

Resolve self/static/$this/parent in parameter types to concrete class names before the type compatibility check, so that class-string is properly validated instead of blanket- suppressed.

The call-site context class is extracted from the call expression: ClassName::method uses the named class, static::/self::/$this-> uses the enclosing class, and parent:: uses its parent. This turns class-string into class-string, letting the existing class-string covariance check flag provably invalid arguments (unrelated classes) while still accepting child classes and siblings.

Closes #273

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, examples/)
• I have updated the config schema (config-schema.json)
• I have added/updated tests to cover my changes
• I fully understand the code that I am submitting (what it does,
  how it works, how it's organized), including any code drafted by an LLM.
• For any prose generated by an LLM, I have proof-read and copy-edited with
  an eye towards deleting anything that is irrelevant, clarifying anything
  that is confusing, and adding details that are relevant. This includes,
  for example, commit descriptions, PR descriptions, and code comments.

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Codecov Report

❌ Patch coverage is 86.11111% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/diagnostics/type_errors/mod.rs	86.11%	5 Missing ⚠️

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