fix: mismatched test case correlation for some rules

[boidolr]

• fix: more informative overall status check
• fix: handle ty errors with special characters

[Copilot]

Pull request overview

This PR updates the pytest-ty plugin’s diagnostic handling to better correlate ty failures with individual test files and to surface a more informative overall status when multiple files fail.

Changes:

• Switch ty check invocation to a structured output format and parse diagnostics via JSON instead of splitting colon-delimited strings.
• Improve the status item error output to include all diagnostic messages (not just file keys).
• Extend tests to cover multiple failing files and a failure case involving diagnostics that previously risked parsing issues.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
tests/test_pytest_ty.py	Adds fixtures and new assertions to ensure multiple-file failures and parsing-sensitive diagnostics are correctly attributed and reported.
src/pytest_ty/plugin.py	Changes ty output format, parses JSON diagnostics into per-file errors, and improves status aggregation output.

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