Address P1 finding: extract_payload now strictly requires the parsed JSON
to be a dict, and validates each field's type and format:
- pr_number: string of digits
- pr_head_sha: 40-char lowercase hex string
- products: alphanumeric with commas/dots/hyphens/underscores only
- run_id: string of digits
- cases_ref: string (may be empty)
validate_run_id also guards against non-string input.
Added tests for: integer/array/string/null JSON, numeric field types,
invalid SHA format, injection in products, missing required fields.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
