docs: clarify makeMessage is a v1-only helper

[cabljac]

Description

makeMessage builds a v1 pubsub.Message, but nothing in its docs says so, and passing its result to wrapV2 fails in a confusing way: the v2 mock factory reads the message's json getter, which eagerly runs JSON.parse(base64Decode(data)) and throws on non-JSON payloads before the handler runs (see #235 for the full diagnosis).

This adds jsdoc to both makeMessage overloads marking the helper as v1-only and showing the CloudEvent-partial patterns to use with wrapV2 instead:

wrapV2(fn)({ data: { message: { json: { hello: 'world' } } } });
wrapV2(fn)({ data: { message: { data: base64String } } });

Docs-only change, no runtime behavior affected.

Fixes #235

[gemini-code-assist]

Code Review

This pull request updates the JSDoc documentation for the makeMessage helper functions in src/providers/pubsub.ts. It adds notes clarifying that these helpers are intended exclusively for v1 functions.pubsub.topic(...).onPublish functions and should not be used with wrapV2 for v2 onMessagePublished functions, providing examples of how to handle v2 functions instead. I have no feedback to provide as there are no review comments.

[wandamora]

LGTM for google review