Include ci/ in coverage source for 100% requirement

[adamtheturtle]

Summary

• Add ci/ to tool.coverage.run.source so ci/test_custom_linters.py is included in the 100% coverage requirement enforced by the coverage job.

Test plan

• CI passes

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Note

Low Risk
Low risk configuration change that expands coverage measurement to include ci/, which may cause CI to fail if those files are not fully exercised.

Overview
Coverage collection is expanded by adding ci/ to tool.coverage.run.source, making CI helper tests (e.g. ci/test_custom_linters.py) part of the 100% coverage gate.

test_tests_collected_once is slightly refactored to use dict.setdefault(...).add(...) when building the tests_to_patterns map (no functional change intended).

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