test(fetch): add full test coverage for fetchHeaders

[dmchaledev]

Summary

fetch.ts — the HTTP layer responsible for fetching remote headers — had 0% test coverage despite being a critical production path. A regression here could cause silent hangs, memory leaks, or wrong headers returned to callers with no failing tests to catch it.

This PR adds 14 focused tests in test/fetch.test.ts that cover every behavior in fetchHeaders:

• Request shape — uses GET, follows redirects, attaches an AbortSignal
• Header normalization — all response header keys are lowercased; empty headers return {}
• Body cleanup — res.body?.cancel() is called to avoid reading/retaining the response body; null body is handled gracefully; errors from cancel() are swallowed (matching the existing /* body may be absent or already closed */ comment)
• Timeout — timeoutMs option is forwarded to setTimeout; defaults to 10 000 ms; the AbortController fires at exactly the right moment; the timer is cleared in both success and error paths (via finally)
• Error propagation — network errors from fetch bubble up unchanged

Coverage impact

Metric	Before	After
Statements	91.8%	98.8%
Branches	96%	97.4%
Functions	~96%	100%
fetch.ts lines	0%	100%

Test plan

• npm test — all 99 tests pass (85 existing + 14 new)
• npm run test:coverage — fetch.ts no longer appears in the uncovered-lines table
• npm run typecheck — no TypeScript errors

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