fix: address PR review — remove token exchange tests, improve logging

- Remove all RFC 8693 token exchange tests (integration, manual, keycloak)
  since Nextcloud doesn't support bearer tokens without upstream patches
- Remove manual impersonation/ADR-004 scripts and their docs
- Clean up token_exchange singleton from integration conftest
- Improve logging in _complete_login_flow_v2_as_user with step-by-step
  [username] prefixed messages matching _complete_login_flow_v2 style
- Remove unnecessary time staggering from all_login_flow_user_tokens;
  concurrent token acquisition works without artificial delays

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

tests/integration/conftest.py

@@ -43,7 +43,6 @@ async def reset_all_singletons():
     # Import all modules with singletons
     import nextcloud_mcp_server.app as app_module
     import nextcloud_mcp_server.auth.client_registry as client_registry_module
-    import nextcloud_mcp_server.auth.token_exchange as token_exchange_module
     import nextcloud_mcp_server.embedding.service as embedding_module
     import nextcloud_mcp_server.observability.tracing as tracing_module
     import nextcloud_mcp_server.providers.registry as registry_module
@@ -63,7 +62,6 @@ async def reset_all_singletons():
         ),
         "tracer": tracing_module._tracer,
         "registry": client_registry_module._registry,
-        "token_exchange_service": token_exchange_module._token_exchange_service,
     }
 
     # Close any open memory streams before reset
@@ -89,7 +87,6 @@ async def reset_all_singletons():
     app_module._vector_sync_state.scanner_wake_event = None
     tracing_module._tracer = None
     client_registry_module._registry = None
-    token_exchange_module._token_exchange_service = None
 
     logger.debug("All singletons reset for test module")
 
@@ -115,4 +112,3 @@ async def reset_all_singletons():
     ) = originals["vector_sync_state"]
     tracing_module._tracer = originals["tracer"]
     client_registry_module._registry = originals["registry"]
-    token_exchange_module._token_exchange_service = originals["token_exchange_service"]