fix: tighten v2 wrapper event typing

[CorieW]

Summary

Touches on #205

• Tighten WrappedV2Function so wrapper arguments must keep CloudEvent shape.
• Preserve flexible data overrides for generated mock payloads and JSON test data.
• Add a compile-time regression check showing wrapped({ data: change }) is valid while raw wrapped(change) is rejected.

Why

V2 wrapped functions accept CloudEvent partials, but the previous DeepPartial<T | object> type let callers pass raw event data. For Firestore change triggers, that makes wrapped(change) compile even though runtime code reads cloudEventPartial.data and falls back to example data.

Validation

• npm run pretest
• npm run lint
• env -u GOOGLE_APPLICATION_CREDENTIALS npm test

[gemini-code-assist]

Code Review

This pull request refactors the WrappedV2Function type in src/v2.ts to use a new WrappedV2CloudEventPartial type, ensuring that V2 wrappers accept CloudEvent partials rather than raw event data. In the test suite, a helper function assertWrappedV2FunctionTypes was added to assert these types. However, this helper function is currently unused, resulting in dead code. It is recommended to wrap these type assertions directly in a standard Mocha it block to ensure they are executed and to prevent potential linter or compiler warnings.