feat: Implement ADR-004 Hybrid Flow with comprehensive integration tests

Implement the ADR-004 Hybrid Flow OAuth pattern where the MCP server
intercepts the OAuth callback to obtain master refresh tokens while
maintaining PKCE security for clients.

## Implementation

### OAuth Routes (ADR-004 Hybrid Flow)
- Add `/oauth/authorize` endpoint: Intercepts client OAuth initiation
- Add `/oauth/callback` endpoint: Receives IdP callback, stores master token
- Add `/oauth/token` endpoint: Exchanges MCP code for client access token
- Implement PKCE code challenge/verifier validation
- Store OAuth sessions with state/challenge correlation

### MCP Server Integration
- Update `setup_oauth_config()` to return client_id and client_secret
- Initialize OAuth context in Starlette lifespan for login routes
- Add OAuth session storage to RefreshTokenStorage
- Configure authlib dependency for OAuth flow management

### Integration Tests
- Create `test_adr004_hybrid_flow.py` with Playwright automation
- Add `adr004_hybrid_flow_mcp_client` session-scoped fixture
- Test MCP session establishment with hybrid flow token
- Test tool execution using stored refresh tokens (on-behalf-of pattern)
- Test persistent access across multiple operations
- All tests passing: ✅ 3 passed in 8.82s

### Documentation
- Update ADR-004 with comprehensive Testing section
- Add integration test commands and coverage details
- Document test implementation and verification steps
- Create TESTING_INSTRUCTIONS.md for manual and automated testing
- Include manual test scripts for reference/debugging

## What This Enables

✅ PKCE code challenge/verifier flow
✅ MCP server intercepts OAuth callback and stores master refresh token
✅ Client receives MCP access token (not master token)
✅ MCP session establishment with hybrid flow token
✅ Tool execution using stored refresh tokens (on-behalf-of pattern)
✅ Multiple operations without re-authentication
✅ Proper token isolation (client never sees master token)

## Testing

Run ADR-004 integration tests:
```bash
uv run pytest tests/server/oauth/test_adr004_hybrid_flow.py --browser firefox -v
```

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

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

uv.lock

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@@ -958,6 +970,7 @@ version = "0.22.7"
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