Add plugin-check-info.json manifest for third-party paths

[faisalahammad]

What?

Closes #760

Add support for a committed plugin-check-info.json manifest that declares bundled third-party paths.

Why?

Plugin Check currently reports warning-level recommendations inside vendor code that plugin authors do not control. This creates noisy results. Errors must remain visible.

How?

Add a shared configuration helper that reads and validates the manifest from the plugin root. After checks run, the shared runner removes warnings whose plugin-relative file path matches a declared third-party path. Errors and findings outside declared paths remain unchanged. The same result filtering is used by CLI and admin AJAX runs.

Path matching uses normalized, relative paths with directory boundaries. Missing or malformed manifests are treated as no configuration.

Testing Instructions

1. Add plugin-check-info.json to a test plugin root:

   {
       "third_parties": [
           "vendor/example-library"
       ]
   }

2. Put code that produces a warning and an error under vendor/example-library/.
3. Put code that produces a warning outside that directory.
4. Run Plugin Check from Tools > Plugin Check and with WP-CLI.
5. Confirm warnings under the declared path are hidden.
6. Confirm errors under the declared path remain visible.
7. Confirm warnings outside the declared path remain visible.
8. Remove or rename the manifest and confirm normal results return.

Automated validation:

• Focused PHPUnit tests pass in wp-env: 28 tests, 69 assertions.
• composer lint passes.
• composer phpstan passes.
• git diff --check passes.

The local PHPUnit command without wp-env was blocked by missing WordPress test environment constants. CodeRabbit follow-up review was rate limited after the initial review completed with no findings.

AI Usage Disclosure

• This PR was created without the help of AI tools
• This PR includes AI-assisted code or content

Used an AI coding assistant to inspect the repository, implement the manifest parsing and result filtering, add tests and documentation, and run validation commands. All changes were reviewed and tested locally.

Screenshots or screencast

Not applicable. This changes check result filtering and does not change the UI layout.

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The following contributors have not linked their GitHub and WordPress.org accounts: @shawn-digitalpoint, @DavidAnderson684.

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Unlinked contributors: shawn-digitalpoint, DavidAnderson684.

Co-authored-by: faisalahammad <faisalahammad@git.wordpress.org>
Co-authored-by: ernilambar <nilambar@git.wordpress.org>
Co-authored-by: swissspidy <swissspidy@git.wordpress.org>

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[faisalahammad]

CI Fix Summary — 1 failure resolved

#	File	Error	Fix
1	includes/Checker/Abstract_Check_Runner.php	PHPMD NPathComplexity (run() NPath 384 > 200)	Extract third-party warning filter to private filter_third_party_warnings()

Tests ✅ (phpmd, lint, phpstan) · Verification ✅ · PHP 7.4 ✅