What kind of change does this PR introduce?

This is a feature implementation.

When using OAuth2Session (requests) or OAuth2Client (httpx) with the client_credentials grant type, the caller must explicitly call fetch_token() before making any resource request, even though all the required metadata (token_endpoint, grant_type, client_id, client_secret) is already configured on the session. This is inconsistent with AssertionSession, which transparently auto-fetches tokens via ensure_active_token().

Before:

from authlib.integrations.requests_client import OAuth2Session

sess = OAuth2Session(
    client_id="my-client",
    client_secret="my-secret",
    token_endpoint="https://auth.example.com/token",
    grant_type="client_credentials",
)

# Must manually fetch before any request — raises MissingTokenError otherwise
sess.fetch_token("https://auth.example.com/token")
resp = sess.get("https://api.example.com/resource")

After:

from authlib.integrations.requests_client import OAuth2Session

sess = OAuth2Session(
    client_id="my-client",
    client_secret="my-secret",
    token_endpoint="https://auth.example.com/token",
    grant_type="client_credentials",
)

# Token is fetched automatically on first request
resp = sess.get("https://api.example.com/resource")

Solution: In request() (and stream() for httpx), when self.token is None and the configured grant_type is "client_credentials", call self.fetch_token() automatically. Since fetch_token() already reads token_endpoint and grant_type from self.metadata when not explicitly provided, the call is simply self.fetch_token() with no arguments needed.

For non-client_credentials grant types, the existing MissingTokenError / OAuthError behavior is preserved unchanged.

Fully backward compatible — sessions that already call fetch_token() explicitly continue to work identically.

Checklist

• The commits follow the conventional commits specification.
• You ran the linters with prek.
• You wrote unit test to demonstrate the bug you are fixing, or to stress the feature you are bringing.
• You reached 100% of code coverage on the code you edited, without abusive use of pragma: no cover
• If this PR is about a new feature, or a behavior change, you have updated the documentation accordingly.

• You consent that the copyright of your pull request source code belongs to Authlib's author.