Improve error messages for invalid target configuration in safe outputs

[Copilot]

When a GitHub Actions expression in target fails to evaluate (e.g., becomes literal "event" instead of a number), safe output handlers fail silently with unclear errors. This happened when users configured target: "${{ github.event }}" which returns an object, not a number.

Changes

Enhanced error detection in safe_output_helpers.cjs:

• Detect failed expression patterns: "event", "[object Object]", or strings containing "github.event"
• Provide context-specific guidance based on handler support (issue-only, PR-only, or both)
• Correctly identify item type in error messages

Before:

Invalid issue number in target configuration: event

After:

Invalid issue number in target configuration: event. It looks like the target contains a 
GitHub Actions expression that didn't evaluate correctly. Make sure you're using a proper 
expression like "${{ github.event.issue.number }}" and that the workflow is running in an 
issue context.

Test coverage:

• Added test verifying issue-only handlers show correct guidance
• Verified handlers supporting both issues and PRs provide both expression examples

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[pelikhan]

@copilot format, lint js, fix tests