ci(mutmut): enable mutate_only_covered_lines to fit in CI budget (#27910)

The mutation-test workflow timed out at the 350-minute job cap when
running whole-folder mutation against litellm/proxy/management_endpoints/
(~30 files, ~1.5 MB of source). Every mutant was running the full
test suite, and mutants were generated for lines no test covers — which
would survive regardless, just wasting compute.

mutmut 3.x's mutate_only_covered_lines setting runs the suite once up
front to compute coverage, then skips mutating uncovered lines. This
cuts the mutant count dramatically and is the right semantic for the
score (no test → no kill possible → uncountable). Per-mutant test
filtering by function name is already automatic in mutmut 3.x; no
external coverage step is needed.

pyproject.toml

@@ -290,6 +290,12 @@ tests_dir = [
 also_copy = [
     "litellm/",
 ]
+# Run the test suite once before mutation to gather line coverage, then skip
+# mutating lines no test exercises. Those mutants would survive regardless
+# (no test hits the line to kill them), so generating them wastes hours of CI.
+# The score now reads as "mutation score over covered code" — pair with a
+# line-coverage number when reporting.
+mutate_only_covered_lines = true
 # Disable rerun/parallel plugins for mutation runs:
 # - pytest-retry triggers an `INTERNALERROR: no option named 'filtered_exceptions'`
 #   when invoked via mutmut's in-process `pytest.main()` call.