test: Replace persistent OAuth client cache with session-scoped fixtures

Remove file-based caching of OAuth client credentials and implement automatic
client lifecycle management for test fixtures.

Changes:
- Add RFC 7592 client deletion function in auth/client_registration.py
- Remove cache_file parameter from _create_oauth_client_with_scopes helper
- Update all OAuth credential fixtures to use yield/finalizer pattern
- Add automatic client cleanup at end of test session (best-effort)
- Remove persistent .nextcloud_oauth_*.json cache files

Benefits:
- No persistent cache files cluttering repository
- Fresh OAuth clients created for each test session via DCR
- Automatic cleanup attempts (RFC 7592 DELETE endpoint)
- Cleaner test environment with proper fixture lifecycle

Note: Client deletion may fail due to Nextcloud authentication middleware
(logged as warning). The key improvement is removing persistent cache files.
OAuth clients may accumulate in Nextcloud but can be cleaned manually.

CLAUDE.md

@@ -327,10 +327,12 @@ OAuth integration tests use **automated Playwright browser automation** to compl
 **OAuth Testing Setup:**
 - **Main fixtures**: `nc_oauth_client`, `nc_mcp_oauth_client` - Use Playwright automation
 - **Shared OAuth Client**: All test users authenticate using a single OAuth client
-  - Stored in `.nextcloud_oauth_shared_test_client.json`
-  - Matches production MCP server behavior
+  - **Created fresh for each test session** via Dynamic Client Registration (DCR)
+  - Matches production MCP server behavior (one client, multiple user tokens)
   - Each user gets their own unique access token
+  - **Automatic cleanup**: Client is registered at session start, deleted at session end (RFC 7592)
   - Implementation: `shared_oauth_client_credentials` fixture in `tests/conftest.py`
+  - **Note**: Client deletion may fail due to Nextcloud middleware (logged as warning). This doesn't affect tests.
 - **Available fixtures**: `playwright_oauth_token`, `nc_oauth_client`, `nc_mcp_oauth_client`
 - **Multi-user fixtures**: `alice_oauth_token`, `bob_oauth_token`, `charlie_oauth_token`, `diana_oauth_token`
 - **Requirements**: `NEXTCLOUD_HOST`, `NEXTCLOUD_USERNAME`, `NEXTCLOUD_PASSWORD` environment variables
@@ -356,7 +358,6 @@ uv run pytest -m oauth -v
   - `mcp-oauth` (port 8001): Uses OAuth authentication - for OAuth-specific testing
 - Start OAuth MCP server: `docker-compose up --build -d mcp-oauth`
 - **Important**: When working on OAuth functionality, always rebuild `mcp-oauth` container, not `mcp`
-- OAuth client credentials cached in `.nextcloud_oauth_shared_test_client.json`
 
 **CI/CD Notes:**
 - Playwright tests run in CI/CD environments