fix(auth): address PR #758 round-2 review

- oauth_login_callback's integrated-mode token-exchange branch now reuses
  the shared discovery cache via get_oidc_discovery (round-2 finding 1).
- AS proxy flow now generates an OIDC nonce in oauth_authorize, stores it
  on ASProxySession, forwards it to the IdP, and passes it as
  expected_nonce to verify_id_token in _oauth_callback_as_proxy
  (round-2 finding 2).
- Consolidate the two parallel discovery caches: oauth_routes' local
  _discovery_cache and _get_cached_discovery are removed; all callers
  now go through token_utils.get_oidc_discovery, which acquires the
  follow_redirects=True knob it needs for Nextcloud installs without
  pretty URLs (round-2 finding 3).
- Demote per-user INFO logs in oauth_tools.py (check_logged_in,
  get_provisioning_status) to DEBUG; the elicitation auth URL is no
  longer logged because it contains a sensitive state token
  (round-2 finding 4).

Also pin nonce binding behaviour with a new unit test that asserts
_oauth_callback_as_proxy forwards session.nonce to verify_id_token, and
update test mocks to track the cache consolidation.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/auth/browser_oauth_routes.py

@@ -407,11 +407,11 @@ async def oauth_login_callback(request: Request) -> RedirectResponse | HTMLRespo
         else:
             # Integrated mode (Nextcloud OIDC)
             discovery_url = oauth_config.get("discovery_url")
-            async with nextcloud_httpx_client() as http_client:
-                response = await http_client.get(discovery_url)
-                response.raise_for_status()
-                discovery = response.json()
-                token_endpoint = discovery["token_endpoint"]
+            # Use the shared 5-minute discovery cache; oauth_login() above
+            # has already populated it for this discovery_url so the
+            # callback should hit the cache rather than re-fetching.
+            discovery = await get_oidc_discovery(discovery_url)
+            token_endpoint = discovery["token_endpoint"]
 
             token_params = {
                 "grant_type": "authorization_code",