refactor: remove RFC 8693 token exchange and Keycloak OAuth implementation

Nextcloud doesn't support OAuth bearer tokens without upstream patches,
making the RFC 8693 token exchange path untestable and dead code.

Removed:
- nextcloud_mcp_server/auth/token_exchange.py (597 lines)
- nextcloud_mcp_server/auth/keycloak_oauth.py (586 lines)
- OAUTH_TOKEN_EXCHANGE deployment mode from AuthMode enum
- get_session_client_from_context() from context_helper.py
- get_session_token() from token_broker.py
- enable_token_exchange / token_exchange_cache_ttl config fields
- oauth_token_exchange_total Prometheus metric
- Keycloak fixture block from tests/conftest.py (~408 lines)
- Token exchange unit tests from test_config_validators.py,
  test_unified_verifier.py, test_management_status_endpoint.py

Preserved:
- Multi-audience OAuth mode (OAUTH_SINGLE_AUDIENCE)
- Login Flow v2 provisioning with elicitation support
- Token broker background token management
- All existing test coverage for non-exchange paths

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/unit/test_management_status_endpoint.py

@@ -37,7 +37,6 @@ def create_mock_settings(
     oidc_issuer: str | None = None,
     vector_sync_enabled: bool = False,
     nextcloud_url: str = "http://localhost",
-    enable_token_exchange: bool = False,
     mcp_client_id: str | None = None,
     mcp_client_secret: str | None = None,
 ):
@@ -49,7 +48,6 @@ def create_mock_settings(
     settings.oidc_issuer = oidc_issuer
     settings.vector_sync_enabled = vector_sync_enabled
     settings.nextcloud_url = nextcloud_url
-    settings.enable_token_exchange = enable_token_exchange
     settings.mcp_client_id = mcp_client_id
     settings.mcp_client_secret = mcp_client_secret
     return settings