perf: speed up mutmut with do_not_mutate, max_stack_depth, and --max-children 4

- Exclude __init__.py, const.py, exceptions.py via do_not_mutate (re-exports,
  constants, and trivial class declarations don't benefit from mutation testing)
- Add max_stack_depth=8 to limit test-to-mutant call stack tracing depth
- Increase --max-children from 2 to 4 (GitHub Actions ubuntu-24.04 has 4 vCPUs)

Co-authored-by: deviantintegral <255023+deviantintegral@users.noreply.github.com>

Author: Copilot

.github/workflows/ci.yml

@@ -26,4 +26,4 @@ jobs:
       - name: Test
         run: uv run pytest --cov=flameconnect --cov-report=term-missing --tb=short
       - name: Mutation test
-        run: uv run mutmut run --max-children 2
+        run: uv run mutmut run --max-children 4

setup.cfg

@@ -1,3 +1,8 @@
 [mutmut]
 paths_to_mutate = src/flameconnect
 also_copy = src
+do_not_mutate =
+    */__init__.py
+    src/flameconnect/const.py
+    src/flameconnect/exceptions.py
+max_stack_depth = 8