test: improve code quality and coverage

[chenrui333]

Summary

• include every production TypeScript file in coverage reporting
• add behavioral coverage for the action entry point and GitHub API wrappers
• type-check tests and replace unsafe test doubles
• narrow internal types and remove confirmed dead state
• enforce stable coverage thresholds

Coverage

Metric	Before	After	Threshold
Statements	89.03%	97.79%	95%
Branches	84.34%	93.64%	90%
Functions	91.66%	96.87%	95%
Lines	88.59%	97.71%	95%

The before value uses explicit src/**/*.ts coverage so unloaded production
files are included.

Runtime impact

No intended user-visible behavior change. Internal changes improve typing,
testability, and safe reporting of non-Error rejections. dist/index.js was
regenerated from the updated source.

Scope

This PR intentionally excludes dependency upgrades and unrelated feature work.