Remove unsafe code from Http KnownHeaders

[MihaZupan]

This also gets rid of the redundant bounds checks for the char variant.
For byte the codegen is the same.

[Copilot]

Pull Request Overview

This PR removes unsafe code from the KnownHeaders implementation and simplifies header candidate lookup by eliminating the redundant IHeaderNameAccessor interface and related types. The key changes are:

• Removal of the unsafe IHeaderNameAccessor interface and its implementations.
• Refactoring of GetCandidate to work with ReadOnlySpan and a new generic constraint.
• Exposure of TryGetKnownHeader methods as public.

Comments suppressed due to low confidence (2)

src/libraries/System.Net.Http/src/System/Net/Http/Headers/KnownHeaders.cs:125

• Ensure that both 'char' and 'byte' types implement INumberBase so the new generic constraint works as expected when using GetCandidate.

private static KnownHeader? GetCandidate<T>(ReadOnlySpan<T> key)

src/libraries/System.Net.Http/src/System/Net/Http/Headers/KnownHeaders.cs:395

• Verify that removing the fixed block for the ReadOnlySpan overload does not introduce any unintended performance regressions or memory safety issues.

KnownHeader? candidate = GetCandidate(name);

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[MihaZupan]

/ba-g test failures are #114769 and #114790