Reduce overhead from using AutomaticDecompression

[MihaZupan]

1. When using AutomaticDecompression, the Accept-Encoding header is practically never touched manually by the user. We can check for this very common case when the header isn't set yet and just add the one value representing all supported methods without more validation.
   • This avoids the allocation of the HttpHeaderValueCollection, the backing List<object>, HeaderStoreItemInfo and enumerators when walking through existing values.
2. When checking if the response was compressed, we can use TryGetValues to avoid the collection allocation if there is no Content-Encoding header.
3. If it is compressed, we can move the content header collection instead of allocating a full copy. Closes DecompressionHandler could move the headers more efficiently #63632.

Simple in-memory benchmark (no real IO)

Method	Toolchain	CompressedResponse	Mean	Error	Ratio	Allocated	Alloc Ratio
GetAsync	\main\corerun.exe	False	897.3 ns	17.91 ns	1.00	1.81 KB	1.00
GetAsync	\pr\corerun.exe	False	674.0 ns	7.99 ns	0.75	1.37 KB	0.75
GetAsync	\main\corerun.exe	True	1,435.9 ns	13.05 ns	1.00	2.53 KB	1.00
GetAsync	\pr\corerun.exe	True	1,212.9 ns	11.70 ns	0.84	1.78 KB	0.70

[Copilot]

Pull request overview

This PR aims to reduce memory allocations when using AutomaticDecompression in SocketsHttpHandler through three main optimizations:

1. Fast-path optimization for the Accept-Encoding header when it hasn't been set yet
2. Using TryGetValues to avoid collection allocation when checking for Content-Encoding headers
3. Moving content headers instead of copying them when creating decompressed content wrappers

Key Changes

• Optimized Accept-Encoding header handling with a pre-computed header value string
• Changed Content-Encoding check to use TryGetValues and work directly with the returned string array
• Modified decompressed content constructors to accept and use the encoding array for header manipulation

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
SocketsHttpHandler.cs	Added defensive check to filter unsupported decompression methods and added constant for supported methods
DecompressionHandler.cs	Implemented fast-path for Accept-Encoding header, optimized Content-Encoding check, modified header handling in decompressed content classes
HttpContent.cs	Added SetHeaders method to support moving headers between content objects