SEP-1166: Add scoped mutmut pilot tooling for app/core/db/utils.py (#1213)

- Pin `mutmut` (^3.6) as a Poetry **dev** dependency and add
`[tool.mutmut]` scoped to `app/core/db/utils.py` with driver
`tests/app/core/db/test_utils.py` (not wired into `make test`,
pre-commit, or CI).
- Document how to re-run the pilot in `CONTRIBUTING.md`; gitignore
mutmut's `mutants/` working copy.
- Implements the tooling half of
[SEP-1166](https://percona.atlassian.net/browse/SEP-1166). Survivor
inventory + kill work tracked in follow-up
[SEP-1662](https://percona.atlassian.net/browse/SEP-1662) (blocked by
SEP-1166).

Author: olucasandrade

.gitignore

@@ -107,6 +107,9 @@ __pypackages__/
 celerybeat-schedule
 celerybeat.pid
 
+# Mutation testing
+mutants/
+
 # Environments
 .env*
 .venv

CONTRIBUTING.md

@@ -197,6 +197,25 @@ Ensure all tests pass before pushing your code:
 make test
 ```
 
+Mutation testing (`mutmut`) is an optional local spike tool, not part of
+`make test`, pre-commit, or CI. Configuration for the pilot scoped to
+`app/core/db/utils.py` lives under `[tool.mutmut]` in `pyproject.toml`. Run it
+from the repo root; it writes a gitignored `mutants/` working copy:
+
+```shell
+poetry run mutmut run           # generate and test mutants
+poetry run mutmut results       # list surviving mutants
+poetry run mutmut show <mutant> # diff for a single mutant
+```
+
+On macOS, export the Homebrew library path first. `mutmut` invokes pytest
+directly, so it bypasses the wrapper that `make test` uses to make WeasyPrint's
+native libraries resolvable:
+
+```shell
+export DYLD_FALLBACK_LIBRARY_PATH="$(brew --prefix)/lib:${DYLD_FALLBACK_LIBRARY_PATH}"
+```
+
 ## Getting Help
 
 If you have any questions or need assistance, feel free to reach out: