Allow Launchplane request JSON file overlays

[shiny-code-bot]

Summary

• Add payload-json-files to the Launchplane request action so workflows can splice parsed JSON files into static request payloads.
• Document the JSON-file overlay pattern for product repos and cover it in action tests.

Verification

• uv run python -m unittest tests.test_launchplane_request_action
• uv run --extra dev ruff check tests/test_launchplane_request_action.py
• uv run --extra dev ruff format --check tests/test_launchplane_request_action.py
• uv run --extra dev mypy tests/test_launchplane_request_action.py
• npx --yes prettier --check .github/actions/launchplane-request/action.yml docs/product-repo-contract.md
• JetBrains changed-file inspection: only existing Node environment weak warnings for require/process in the bundled action file