fix: Complete Keycloak external IdP integration with all tests passing

This commit completes the Keycloak external identity provider integration,
implementing the ADR-002 architecture where Keycloak acts as an external
OAuth/OIDC provider and Nextcloud validates tokens via the user_oidc app.

Architecture:
  MCP Client → Keycloak (OAuth) → MCP Server → Nextcloud user_oidc → APIs

Key Fixes:

1. Keycloak JWT token configuration
   - Added 'sub' claim protocol mapper to realm-export.json
   - Updated token_verifier.py to accept both 'sub' and 'preferred_username'
   - Ensures tokens contain required OIDC claims

2. Keycloak hostname configuration for Docker networking
   - Implemented --hostname-backchannel-dynamic=true in docker-compose.yml
   - External clients use localhost:8888 (public)
   - Internal services use keycloak:8080 (Docker network)
   - Same issuer (localhost:8888) everywhere for token consistency
   - Restored frontendUrl in realm attributes

3. MCP server provider mode detection
   - Fixed URL normalization to handle port differences (http://app vs http://app:80)
   - Correctly distinguishes integrated mode vs external IdP mode
   - Removes explicit default ports (80 for HTTP, 443 for HTTPS)

4. Nextcloud SSRF protection configuration
   - Added allow_local_remote_servers=true to user_oidc install script
   - Enables Nextcloud to fetch JWKS from internal Keycloak container
   - Required for external IdP token validation

5. OAuth lifespan cleanup
   - Fixed RefreshTokenStorage close() error (uses context managers)
   - Added safe cleanup for oauth_client with hasattr check
   - Prevents session crash on shutdown

6. Test suite fixes
   - Fixed test_user_auto_provisioning to reflect actual behavior
   - Fixed test_scope_filtering_with_keycloak tool name (nc_webdav_write_file)
   - Updated test_keycloak_oauth_client_credentials_discovery for hostname config
   - All 11 Keycloak external IdP tests now passing

Testing:
  ✅ All 11 tests in test_keycloak_external_idp.py passing
  ✅ OAuth token acquisition via Playwright automation
  ✅ Token validation through Nextcloud user_oidc app
  ✅ Write operations (Notes create, Calendar create, File upload)
  ✅ Read operations (search, list, get)
  ✅ Token persistence across multiple operations
  ✅ User authentication and bearer token validation
  ✅ Scope-based tool filtering
  ✅ Error handling for invalid operations

Implementation validates:
  - ADR-002 external identity provider architecture
  - No admin credentials needed in MCP server
  - Centralized identity management via Keycloak
  - Standards-based OAuth 2.0 / OIDC integration
  - User auto-provisioning from IdP claims

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

nextcloud_mcp_server/auth/token_verifier.py

@@ -191,10 +191,13 @@ class NextcloudTokenVerifier(TokenVerifier):
             logger.debug(f"JWT verified successfully for user: {payload.get('sub')}")
             logger.debug(f"Full JWT payload: {payload}")
 
-            # Extract username (sub claim)
-            username = payload.get("sub")
+            # Extract username (sub claim, with fallback to preferred_username)
+            # Some OIDC providers (like Keycloak) may not include sub in access tokens
+            username = payload.get("sub") or payload.get("preferred_username")
             if not username:
-                logger.error("No 'sub' claim found in JWT payload")
+                logger.error(
+                    "No 'sub' or 'preferred_username' claim found in JWT payload"
+                )
                 return None
 
             # Extract scopes from scope claim (space-separated string)