Add integration test for JWT auth pipeline

[rdwj]

The current test_auth.py verifies that configure_auth() returns a JWTVerifier when env vars are set, but doesn't test actual token verification through the FastMCP auth pipeline.

Acceptance criteria:

• Test that a tool with auth=require_scopes("admin") rejects unauthenticated requests
• Test that a valid JWT with the correct scope is accepted
• Test that a valid JWT with wrong scopes is rejected

Context: identified in FastMCP 3.x migration retro.