Remove the dead HttpContext.Items read from ParseContentType

[glen-84]

Summary

• ParseContentType opened by looking up HttpContext.Items["RequestContentType"], a key nothing has written since Add Fusion Transport Layer #8410 moved the method into HttpContextExtensions. The lookup never hit, and on a request where nothing else had touched Items it created the Items feature: 48 bytes on every POST, QUERY, and multipart request on Kestrel.
• Writing the key instead would not pay off. A cache hit (about 11 ns) costs more than the parse it skips (4 to 10 ns, at most two case-insensitive span comparisons), no request calls the method more than twice, and the write allocates the backing dictionary (264 bytes per request).
• No response changes. The lookup also honored the key when application code set it before MapGraphQL, which was never documented.

Test plan

• HotChocolate.AspNetCore.Tests and HotChocolate.Fusion.AspNetCore.Tests pass on net10.0. The Fusion gateway runs the same request middlewares.
• Allocations and per-call timings were measured inside Kestrel requests on .NET 11 RC 1.

[Copilot]

Copilot review overview

🟢 Approval recommended

No unresolved review issues remain.

Review effort: Lite
Findings: None

What changed in this PR

Removes the unused HttpContext.Items lookup from ParseContentType, reducing request allocations while preserving content-type handling.

Changes:

• Parses ContentType directly.
• Preserves JSON, multipart, and query handling.

File	Summary
src/​HotChocolate/​AspNetCore/​src/​AspNetCore.Pipeline/​Extensions/​HttpContextExtensions.cs	Removes the obsolete Items lookup.

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