dart_mcp_server: add `args` parameter to `launch_app` tool (#354)

Fixes #342

## Summary

- Adds an `args` list parameter to the `launch_app` tool, allowing callers to forward arbitrary extra flags to `flutter run` (e.g. `--flavor dev`, `--dart-define-from-file env.json`, `--release`).
- Managed flags (`--print-dtd`, `--machine`, `--device-id`, `--target`, `-d`, `-t`) are explicitly blocked with a clear error message — both in exact form and equals-syntax (e.g. `--target=lib/main.dart`).
- Updated CHANGELOG and GEMINI.md.

## Test plan

- [ ] `launch_app forwards additional args to flutter run` — verifies args are appended to the command
- [ ] `launch_app works when args is an empty list` — verifies no regression on omitted/empty args
- [ ] `launch_app forwards flavor args` — verifies `--flavor` works end-to-end
- [ ] `launch_app rejects managed args in args list` — verifies `--device-id` in `args` returns `isError: true`
- [ ] `launch_app rejects managed args with equals syntax` — verifies `--target=lib/alt_main.dart` is also caught

All 14 tests in `flutter_launcher_test.dart` pass. Run with:
```
cd pkgs/dart_mcp_server && dart test test/tools/flutter_launcher_test.dart
```