fix(auth0): stop rediscovering the jwks uri on every verifier

[chiruu12]

Fixes #6781

__init__ re-ran OIDC discovery and assigned it to self.jwks_uri. Nothing reads that attribute. _verify_bearer_token takes its signing key from the module level jwks_client, which was built from the discovery that already ran at import, so the second lookup could never return anything the first one had not.

The attribute now carries that same value. The second request is gone, and jwks_uri is set to None alongside jwks_client when AUTH0_DOMAIN is unset so the name exists on both paths.

It adds up. get_auth_verifier builds a new verifier per call and sits in 190 route dependencies, which FastAPI evaluates at import, so AUTH_TYPE=AUTH0 meant 190 discarded discovery requests at startup with a 10 second timeout apiece.

Three tests. Import discovers exactly once, three constructions add no further requests, and the constructed verifier still carries the discovered URI. Only the middle one fails on the parent commit; the other two pass either way and are controls, not reproductions.

tests/test_auth0_authverifier.py: 3 passed. tests/test_auth.py tests/test_auth_new.py: 27 passed. ruff check clean.

This does not fix the unit-tests timeouts on #6716. Those come from the import time call being unmocked for MULTI_TENANT in tests/fixtures/client.py, which is a separate thing and I did not touch it here.

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