Tests: gzip Accept-Encoding HTAB whitespace

[louzt]

Adds gzip_accept_encoding_htab.t, a focused regression test for HTAB
handling in the Accept-Encoding request header. Per RFC 9110 §5.6.2 /
§5.6.3, the surrounding field grammar defines optional whitespace as
SP / HTAB, and Vadim's nginx#1577 already taught the multi-header parser
to accept HTAB. The gzip_accept_encoding helper in
ngx_http_core_module.c carries its own boundary check, so it must be
updated to match.

The gap is the single-character path that runs after the gzip token
has been matched: the switches in ngx_http_gzip_accept_encoding()
only carry case ' ' and exit on the bare comma that ends the list,
so HTAB falls into the default branch and the encoding is rejected.

The test covers four baseline cases that document the current
behaviour (SP, no whitespace, HTAB after comma, malformed input
without comma) and three TODO cases that document the gap and will
succeed once the companion nginx#1627 lands.

The malformed-input case (gzip\tdeflate, no comma) is held as a
baseline regression: RFC 9110 §5.6.2 requires comma separators
between elements, and whitespace alone is not enough.

Testing

Verified against a patched and an unpatched binary:

• unpatched (current master): 9 tests overall, 3 TODO cases fail as
  expected, 4 baseline cases pass, suite reports PASS.
• patched (companion C PR applied): 9/9 pass, all 3 TODO cases now
  succeed.
• gzip.t regression suite: 10/10 pass on the patched binary.

# Unpatched master
1..9
ok 1 - gzip enabled with SP between tokens
ok 2 - gzip enabled with no whitespace between tokens
ok 3 - gzip enabled with HTAB after comma
ok 4 - malformed Accept-Encoding without comma is rejected
not ok 5 - gzip enabled with HTAB before comma # TODO
not ok 6 - gzip enabled with HTAB before q-value # TODO
not ok 7 - gzip enabled with HTAB after q-value before comma # TODO
ok 8 - no alerts
ok 9 - no sanitizer errors
All tests successful.

# Patched companion
1..9
ok 1 - gzip enabled with SP between tokens
ok 2 - gzip enabled with no whitespace between tokens
ok 3 - gzip enabled with HTAB after comma
ok 4 - malformed Accept-Encoding without comma is rejected
ok 5 - gzip enabled with HTAB before comma
ok 6 - gzip enabled with HTAB before q-value
ok 7 - gzip enabled with HTAB after q-value before comma
ok 8 - no alerts
ok 9 - no sanitizer errors
All tests successful.

Companion C PR

nginx#1627: HTTP: HTAB support in Accept-Encoding list parsing
(5-line surgical patch to ngx_http_core_module.c).

References

• nginx#1577 — multi-header parser rework, merged 2026-08-05;
  Vadim explicitly carved ngx_http_gzip_accept_encoding() out
  because the helper carries its own boundary check.
• RFC 9110 §5.6.2 — optional whitespace definition (OWS = SP / HTAB).
• RFC 9110 §5.6.3 — Accept-Encoding field grammar (#rule).