Include client_id in client_credentials token request body

[rameshreddy-adutla]

Summary

Include client_id in the token request body for ClientCredentialsOAuthProvider.

Problem

ClientCredentialsOAuthProvider._exchange_token_client_credentials() was missing client_id in the token_data dict. Per RFC 6749 §2.3.1, when using client_secret_post authentication, both client_id and client_secret must be in the request body.

The prepare_token_auth() method only adds client_secret for client_secret_post, so client_id was never sent — causing authentication failures with OAuth providers that require it.

Note: even for client_secret_basic, having client_id in the body is harmless per RFC 6749, and many providers expect it.

Fix

Add client_id from self._fixed_client_info to the initial token_data dict, consistent with how other token exchange methods in the SDK include it (lines 336, 389, 436 of oauth2.py).

Testing

All 13 existing tests pass. Added explicit client_id assertions to both test_exchange_token_client_credentials and test_exchange_token_without_scopes tests.

Fixes #2128