Fix completion and validation issues in action.yml

[ericsciple]

Follow-up to PR:

• Context-aware autocomplete and validation for action.yml runs section #289

What this fixes

Autocomplete was broken inside composite action steps. When you typed inside a step and triggered autocomplete, nothing showed up. Now you correctly get suggestions like run, uses, shell, etc.

Duplicate error messages for missing required fields. When a required field was missing (like main for Node.js actions), users saw two error messages - one generic schema validation error, and one custom error with a clear explanation. Now they only see the custom one.

For example, with using: node24 but no main:

• Before: Two errors shown
  • Schema: "There's not enough info to determine what you meant. Add one of these properties: args, entrypoint, image, main, ..."
  • Custom: "'main' is required for Node.js actions (using: node24)"
• After: Only the custom error is shown

[Copilot]

Pull request overview

This PR fixes two issues with GitHub Actions action.yml language services: broken autocomplete inside composite action steps and duplicate error messages for missing required fields.

• Refactored validation logic to filter out redundant schema errors when custom, more specific error messages are provided
• Fixed completion filtering logic to correctly detect when the user is completing inside nested structures like composite action steps
• Added comprehensive test coverage for both fixes

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
languageservice/src/validate-action.ts	Refactored validateRunsKeys to validateRunsKeysAndFilterErrors which now filters redundant schema errors in-place when custom diagnostics provide better error messages
languageservice/src/validate-action.test.ts	Added 8 new test cases covering duplicate error filtering scenarios including missing required keys, different action types, and edge cases
languageservice/src/complete.ts	Simplified the logic for detecting nested completion contexts by checking if runsMapping appears before the end of the path, rather than finding the last mapping
languageservice/src/complete-action.test.ts	Added test verifying that step-level keys (run, uses, shell, etc.) are available when completing inside composite action steps

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