Tighten coverage to 90% on production code only

Coverage source now excludes tests/ so the reported percentage
reflects production code, not test boilerplate. The fail_under
threshold moves from 50% to 90%, matching the actual 98% coverage
with room for new code that isn't immediately testable.

Author: audreyfeldroy

pyproject.toml

@@ -77,12 +77,12 @@ select = [
 [tool.coverage.run]
 branch = true
 parallel = true
-source = ["src/", "tests/"]
+source = ["src/"]
 
 [tool.coverage.report]
 show_missing = true
 skip_covered = true
-fail_under = 50
+fail_under = 90
 exclude_also = [
     "if TYPE_CHECKING:",
     "if typing.TYPE_CHECKING:",