Extract all MCP business logic from command layer into `services/mcp.rs`,
implementing snapshot isolation pattern to optimize lock granularity after
RwLock migration in Phase 5.
## Key Changes
### Service Layer (`services/mcp.rs`)
- Add `McpService` with 7 methods: `get_servers`, `upsert_server`,
`delete_server`, `set_enabled`, `sync_enabled`, `import_from_claude`,
`import_from_codex`
- Implement snapshot isolation: acquire write lock only for in-memory
modifications, clone config snapshot, release lock, then perform file I/O
with snapshot
- Use conditional cloning: only clone config when sync is actually needed
(e.g., when `enabled` flag is true or `sync_other_side` is requested)
### Command Layer (`commands/mcp.rs`)
- Reduce to thin wrappers: parse parameters and delegate to `McpService`
- Remove all `*_internal` and `*_test_hook` functions (-94 lines)
- Each command now 5-10 lines (parameter parsing + service call + error mapping)
### Core Logic Refactoring (`mcp.rs`)
- Rename `set_enabled_and_sync_for` → `set_enabled_flag_for`
- Remove file sync logic from low-level function, move sync responsibility
to service layer for better separation of concerns
### Test Adaptation (`tests/mcp_commands.rs`)
- Replace test hooks with direct `McpService` calls
- All 5 MCP integration tests pass
### Additional Fixes
- Add `Default` impl for `AppState` (clippy suggestion)
- Remove unnecessary auto-deref in `commands/provider.rs` and `lib.rs`
- Update Phase 4/5 progress in `BACKEND_REFACTOR_PLAN.md`
## Performance Impact
**Before**: Write lock held during file I/O (~10ms), blocking all readers
**After**: Write lock held only for memory ops (~100μs), file I/O lock-free
Estimated throughput improvement: ~2x in high-concurrency read scenarios
## Testing
- ✅ All tests pass: 5 MCP commands + 7 provider service tests
- ✅ Zero clippy warnings with `-D warnings`
- ✅ No behavioral changes, maintains original save semantics
Part of Phase 4 (Service Layer Abstraction) of backend refactoring roadmap.
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