Annotate Authlib introspect_token

[popsiclelmlm]

Summary

• Annotate Authlib's introspect_token method so calls are no longer treated as untyped.
• Add a regression test covering both sync and async httpx OAuth2 clients.

Reproduction

• AsyncOAuth2Client.introspect_token(...) was reported in introspect_token raises no-untyped-call violation in 1.6.6.20251220 #15164 as triggering mypy's no-untyped-call check after the prior Incomplete alias was removed.

Root cause

• authlib.oauth2.client.OAuth2Client.introspect_token had no annotations in the stub.
• The httpx integration subclasses inherit that runtime method; the async variant returns the awaitable produced by AsyncClient.post, while the sync variant returns the response directly.

Changes

• Add explicit parameter and return annotations to the base Authlib OAuth2 client method.
• Override the httpx sync/async OAuth2 client signatures to model the sync response vs async awaitable response behavior.
• Add a regression testcase for introspect_token usage.

Tests

• PATH=/tmp/typeshed-authlib-introspect-venv/bin:$PATH /tmp/typeshed-authlib-introspect-venv/bin/python tests/runtests.py stubs/Authlib
• git diff --check

Screenshots/Logs

• N/A

Closes #15164

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉