fix: improve error messages for invalid rule files

[sarthakNITT]

Description

• Improved validation logic to detect when a file in .continue/rules fails to parse.
• Replaced the generic "Failed to convert prompt file" error with a more specific "Failed to parse rule definition" message for rule files.
• Included the actual YAML parser error details in the message to help users debug syntax issues (e.g., missing commas, flow sequence errors).

Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

issue #9548

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Summary by cubic

Improved error messages for invalid rule files in rules directories. Users now see clear YAML parsing details to quickly fix syntax issues.

• Bug Fixes
  • Detect rule files by path and tailor the error message.
  • Replace the generic prompt error with “Failed to parse/process rule definition …”.
  • Include YAML parser details (e.g., flow sequence errors) in the message.

^{Written for commit 9694b24. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

[sarthakNITT]

@RomneyDa Sure.

[RomneyDa]

Thanks for the contribution!