Make content sniffing trim-safe

[quinnj]

HTTP.sniff, and servecontent without a content_type (or a name with a known extension), failed juliac --trim=safe on Julia 1.13.1 with three verifier errors:

• HTTP._sniff_match(::Any, data, ::Int64)
• HTTP._sniff_content_type(::Any)
• convert(String, ::Any)

Root cause: _SNIFF_SIGNATURES was an Any[...] vector, so each call in the sniff loop dispatched at runtime. The table holds six signature types, more than inference will union-split (four), so giving it a concrete element type alone isn't enough.

Fix: the table is now Union{_SniffHTMLSig,_SniffMasked,_SniffExact}[...]. The three data-free signatures that always ran last (MP4, JSON, text) move to a tuple that sniff checks after the table. Both loops have three types, so dispatch is static. Signature order and results are unchanged.

Coverage: test/http_trim_open_fileserver.jl now calls servecontent with bytes and no content type, plus HTTP.sniff on a string, and checks the results. That workload runs the built executable, so the content types are also checked at runtime.

Found while reviewing #1397, whose router workload passes content_type explicitly to avoid this gap.

Verification (Julia 1.13.1, macOS arm64)

• Trim workload with the fix: verifier clean, executable exits 0 (6/6).
• Same workload with only the src change reverted: 12 verifier errors, all the three sniff calls above (once each for the Vector and SubArray inputs, via both callers).
• Old vs new sniff on 25,371 inputs (every signature with whitespace, case, and suffix variants, plus random bytes up to 600 long), as both Vector and view: 0 mismatches, 27 distinct content types.
• http_forms_tests.jl, http_multipart_tests.jl, http_server_http1_tests.jl, public_api_tests.jl: 909/909 pass.

Not in this PR

servecontent(request, io) with an IO source and no size still fails trim: _source_size formats a caught error with sprint(showerror, err), which adds about 18 verifier errors. That's a separate change.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.79%. Comparing base (52ba567) to head (d2d982d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1398      +/-   ##
==========================================
- Coverage   90.80%   90.79%   -0.02%     
==========================================
  Files          31       31              
  Lines       12835    12838       +3     
==========================================
+ Hits        11655    11656       +1     
- Misses       1180     1182       +2     

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