Add cov-mark coverage for type-checker error dedup

[kaankacar]

Added cov-mark coverage for type-checker error dedup

• Add cov-mark as a workspace/test dependency.
• Mark the first and duplicate branches in TypeChecker::push_error_dedup.
• Extend the error-recovery test to assert the duplicate path is exercised with cov_mark::check!.

[kaankacar]

Fixes #51

[Copilot]

Pull request overview

This PR adds test coverage instrumentation using the cov-mark library to verify that the type-checker's error deduplication logic is properly exercised. The changes ensure both the "first occurrence" and "duplicate skipped" code paths are tested.

Changes:

• Added cov-mark as a workspace dependency and included it in the type-checker and tests packages
• Instrumented the error deduplication logic with coverage marks for both first occurrence and duplicate detection
• Simplified the deduplication test case and added assertions to verify both code paths are exercised

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
Cargo.toml	Added cov-mark 2.2.0 as a workspace dependency
core/type-checker/Cargo.toml	Added cov-mark dependency to type-checker package
core/type-checker/src/type_checker.rs	Instrumented error deduplication branches with cov_mark::hit! calls
tests/Cargo.toml	Added cov-mark dependency to tests package
tests/src/type_checker/error_recovery.rs	Added cov_mark::check! assertions and simplified test case

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[Copilot]

The cov_mark::check! call is placed before the code that should trigger the coverage mark. This will verify the mark was hit in a previous test run rather than during this specific test execution. Move the check after try_type_check(source) to properly verify the mark is hit during this test.