feat(gateway): IPIP-523 format query over Accept header

[lidel]

this change simplifies precedence rules by making the ?format= URL query parameter always take priority over the Accept HTTP header when both are present.

in practice, this is largely compatible with existing browser use cases since browsers send Accept headers with wildcards which were already treated as non-specific. prioritizing ?format= also ensures deterministic HTTP caching behavior, protecting against CDNs that commingle different response types under the same cache key.

the only breaking change is for edge cases where a client sends both a specific Accept header and a different ?format= value. previously Accept would win, now ?format= wins. this scenario is rare and arguably represents client misconfiguration. when detected, gateway returns HTTP 400 to signal the ambiguity.

• specs: IPIP-0523: Prefer format request param over Accept header specs#523
• tests: test!(ipip-523/v0.9): prefer format query param over accept header gateway-conformance#252

TODO

• Kubo PR which uses this PR and tests from test!(ipip-523/v0.9): prefer format query param over accept header gateway-conformance#252 and the CI is green there

[codecov]

Codecov Report

❌ Patch coverage is 96.29630% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 61.10%. Comparing base (63b6a19) to head (d17c0a2).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
gateway/handler_car.go	83.33%	0 Missing and 1 partial ⚠️

@@            Coverage Diff             @@
##             main    #1074      +/-   ##
==========================================
+ Coverage   61.06%   61.10%   +0.03%     
==========================================
  Files         264      264              
  Lines       26221    26237      +16     
==========================================
+ Hits        16013    16031      +18     
+ Misses       8527     8526       -1     
+ Partials     1681     1680       -1     

Files with missing lines	Coverage Δ
gateway/handler.go	78.62% <100.00%> (+0.71%)	⬆️
gateway/handler_car.go	79.71% <83.33%> (-1.45%)	⬇️

... and 5 files with indirect coverage changes

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[lidel]

CI with end-to-end tests at

• ipfs/kubo#11086

are green.

AFAIK this PR is a low risk change (see Compatibility section in IPIP-523)

[lidel]

Switched to gateway-conformance@v0.9, merging.

This is low risk change, we will test on gateway staging with Kubo 0.40.0-rc1.