Improve HTTP header parser recovery performance

[javiercn]

Fixes #68305

Summary

• report consumed malformed spans from additional HTTP header parser failure paths
• preserve forgiving-list recovery of subsequent valid values
• add regression coverage for shared separators, media types, Set-Cookie, and quality values
• add BenchmarkDotNet coverage through 32,768-character inputs

Validation

• dotnet test .\src\Http\Headers\test\Microsoft.Net.Http.Headers.Tests.csproj -c Release --no-restore
• dotnet build .\src\Http\Http\perf\Microbenchmarks\Microsoft.AspNetCore.Http.Microbenchmarks.csproj -c Release --no-restore -p:UseIisNativeAssets=false
• BenchmarkDotNet validation for HeaderParserListBenchmarks

[Copilot]

Pull request overview

This PR improves the performance of forgiving (non-strict) HTTP header list parsing by ensuring element parsers report how much malformed input they already inspected, allowing the shared recovery loop to skip past invalid spans in O(1) rather than repeatedly re-scanning the same characters. It also adds targeted regression tests and microbenchmarks to validate both correctness (recovery of subsequent valid values) and performance at realistic worst-case input sizes.

Changes:

• Propagate “consumed malformed span” lengths from additional failure paths in BaseHeaderParser<T>, MediaTypeHeaderValue, SetCookieHeaderValue, and StringWithQualityHeaderValue.
• Add regression tests covering recovery of a valid value after malformed separators, media types (missing / / subtype), Set-Cookie (missing =), and invalid quality suffixes.
• Add BenchmarkDotNet coverage for these scenarios up to 32,768-character inputs.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Http/Http/perf/Microbenchmarks/HeaderParserListBenchmarks.cs	Adds microbenchmarks for key malformed-list recovery scenarios across increasing input sizes.
src/Http/Headers/test/StringWithQualityHeaderValueTest.cs	Adds regression test ensuring invalid quality suffix recovery preserves following valid value.
src/Http/Headers/test/SetCookieHeaderValueTest.cs	Adds regression test ensuring missing = recovery preserves following valid Set-Cookie.
src/Http/Headers/test/MediaTypeHeaderValueTest.cs	Adds regression tests for recovery after missing / and missing subtype patterns.
src/Http/Headers/test/EntityTagHeaderValueTest.cs	Adds regression coverage validating separator-skipping recovery avoids repeated parsing work.
src/Http/Headers/src/StringWithQualityHeaderValue.cs	Reports consumed malformed span when quality parsing fails after ;.
src/Http/Headers/src/SetCookieHeaderValue.cs	Reports consumed malformed span when cookie name is missing =.
src/Http/Headers/src/MediaTypeHeaderValue.cs	Reports consumed malformed span for missing / and missing subtype cases via null mediaType signaling.
src/Http/Headers/src/BaseHeaderParser.cs	Reports consumed malformed span when list separator/whitespace was scanned before element parsing returns 0.

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