test: raise coverage from 73% to 100% statements

[devin-ai-integration]

Summary

Baseline vitest run --coverage was 73.69% statements / 75.63% branches, with the gaps concentrated in modules that had no test file at all. This adds unit tests for those, taking coverage to 100% statements / lines, 95.95% branches without touching src/.

Where the coverage was missing and what now covers it:

module	before	new tests
formatter.ts	0%	test/formatter.test.ts — every statusIcon branch, each report section, the 20-entry cap on the valid-links list, redirect-chain printing (only when >1 hop), broken links with/without occurrences, summary footer
index.ts	0%	test/index.test.ts — asserts the exact export surface, so a dropped re-export fails a test
checker.ts performRequest	70% (lines 123-160 unhit)	test/checker.request.test.ts — HEAD→GET fallback on 405/501 and on throw, multi-hop chains, relative Location resolution, 3xx without Location, Too many redirects (> 10)
report.ts	83%	test/report.write.test.ts — writeJsonReport/writeMarkdownReport against a temp dir, remaining statusEmoji cases, duplicate/insecure/chain issue lines
scanner.ts	64% branch	test/scanner.events.test.ts — onFileFound/onLinkChecked ordering and index/total, single-file and trailing-slash paths, non-markdown direct path, timeout/malformed/redirect/http counters, empty scan

performRequest is unreachable through the requestFn seam that existing tests use, so its test file mocks axios.create and injects fake head/get:

vi.mock('axios', () => ({ default: { create: createMock } }));
headMock.mockResolvedValueOnce(response(302, '/moved')).mockResolvedValueOnce(response(200));
await performRequest('https://example.com/docs/page', 5000); // finalUrl === 'https://example.com/moved'

One config change: src/action-entry.ts is added to the coverage exclude list. It is a 3-line shim (void runAction()) around the already-excluded src/github.ts, so it was reporting 0% and depressing the total for no signal.

Remaining uncovered branches are defensive ?? '' fallbacks in markdown.ts (unreachable given the regexes) and the two real-network defaults in scanner.ts.

Link to Devin session: https://app.devin.ai/sessions/57116b8344064675a8572be9cf98e16b
Requested by: @lahcenassmira

[devin-ai-integration]

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