Add testing to describe action

[nathro]

Handle multi-line custom guidelines in Describe PR action

Jira: [ENG-XXXX]

Summary

Add support for passing and safely parsing multi-line custom guidelines from the workflow into the action's context used for PR description generation. This makes the Describe PR action robust when teams provide project-specific guidance.

Changes

• Workflow (.github/workflows/describe-pr.yml): Provide a multiline custom_guidelines example to the action, demonstrating typical project rules (e.g., Jira references, migrations/backfills highlighting).
• Action (action.yml): Write the context output using the multiline GITHUB_OUTPUT syntax (<<EOF) so JSON from jq that includes newlines is preserved when CUSTOM_GUIDELINES is set; otherwise default to {}.

Impact

• Ensures custom, multi-line guidelines are reliably delivered to the action without truncation or JSON parsing issues.
• Enables stronger, project-specific PR descriptions (e.g., enforcing Jira references and calling out migrations/backfills).
• No breaking changes and no new dependencies.

Database Migrations

• None

Backfills

• None

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