Fix: Raise helpful TypeError for invalid list 'data'

[mahdirahimi1999]

Fixes #3471

Summary

This PR fixes a misleading error that occurred when passing an invalid list (e.g., a list of dictionaries) to the data parameter.

Previously, AsyncClient would raise a confusing RuntimeError instead of a TypeError that accurately described the problem. This change introduces a validation check in BaseClient to catch this common mistake early and raises a helpful TypeError. The new error message guides the user to use the json parameter for JSON arrays, ensuring consistent behavior between both sync and async clients.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly. (N/A: This change improves an error message and does not require a documentation update.)

[Zaczero]

P.S. I am just a random user browsing through open PRs; I may be completely wrong.

[Zaczero]

This seems quite inefficient. Also, logically it should be the inverse as you want to check for 2-item collections: For form data, use a dictionary or a list of 2-item. So whitelist instead of blacklist. For example, list[None] is not raising a TypeError here, and much more.

[Zaczero]

The PR says: Raise helpful TypeError for invalid list 'data', but you seem to add a brand new set of logic for handling new request data type.

[lovelydinosaur]

Thanks tho no. Note that the upcoming httpx 1.0 pre-release comprehensively deals with getting to a much cleaner & more minimally typed API.