refactor: improve coverage handler typing and registry

[ideepu]

Introduce BaseCoverage and generic handlers, consolidate coverage loading into get_coverage, and add tests for compute_contiguous_groups.

[ideepu]

Coverage report

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File	Statements	Missing	Coverage	Missing stmts
codecov
config.py				115
github.py
github_client.py				128, 130
main.py				121-122
codecov/coverage
base.py				75-77, 102-104
jest.py				65-69, 73-74, 77-88, 94-106, 112-126, 145-159, 175-179, 194-212, 217
Project Total

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