Order the gzip hijack error path so failures cannot corrupt the body

[paskal]

Follow-up to #55, addressing the error-path ordering umputun raised there.

hijack closed the gzip stream, discarded the Close error, and released the writer to the pool, all before attempting the hijack:

if w.gz != nil {
    _ = w.gz.Close()
    gzPool.Put(w.gz)
    w.gz = nil
}
conn, rw, err := h.Hijack()

Two things followed from that.

A failing Close was invisible. It means truncated gzip output, and a hijack succeeding immediately after hid the fact. The error is returned now, wrapped.

A failing Hijack could corrupt the response. hijacked stayed false while w.gz was already nil, so a handler that carried on writing took the w.ResponseWriter.Write path and appended raw bytes to a body already carrying Content-Encoding: gzip. On that path the writer now stays attached, and because it is closed a further write fails with flate: closed writer instead. Verified rather than assumed: a closed gzip.Writer returns that error and leaks nothing into the output.

The writer is released only once Hijack has actually taken the connection. On the failure path the deferred close still returns it to the pool, which is safe because Reset clears the closed flag, and a second Close on an already-closed writer returns nil.

Three cases in the new test, covering the failing close, the failed hijack followed by a write, and the successful hijack releasing the writer. The first two fail on the current code.

Narrow in practice, as noted on #55 — Hijack failing after the type assertion succeeded is mostly a double hijack — but the ordering is strictly safer and costs nothing.

[coveralls]

Coverage Report for CI Build 32201953393

Coverage increased (+0.2%) to 97.186%

Details

• Coverage increased (+0.2%) from the base build.
• Patch coverage: 14 of 14 lines across 1 file are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	1706
Covered Lines:	1658
Line Coverage:	97.19%
Coverage Strength:	47.32 hits per line

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