Support reading compressed grpc-web trailing headers

[JamesNK]

Fixes #2593

[Copilot]

Pull Request Overview

This PR adds support for reading compressed grpc-web trailing headers, fixing issue #2593. The implementation enables decompression of gRPC-Web trailers that are compressed using gzip or deflate encoding.

• Moved the GetHeaderValue method from GrpcProtocolHelpers to HttpRequestHelpers for shared usage
• Added compression support for gRPC-Web trailing headers by detecting compressed trailers and decompressing them using appropriate codecs
• Updated constructor signatures to pass HttpResponseMessage through the call chain to access compression headers

Reviewed Changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/Grpc.Net.Client.Web/Internal/GrpcWebResponseStream.cs	Added compressed trailer detection and decompression logic
src/Grpc.Net.Client.Web/Internal/CompressionHelpers.cs	New helper class for compression/decompression operations
src/Grpc.Net.Client.Web/Internal/GrpcWebResponseContent.cs	Updated constructor to pass HttpResponseMessage
src/Grpc.Net.Client.Web/GrpcWebHandler.cs	Updated to pass response message to content wrapper
src/Shared/HttpRequestHelpers.cs	Moved GetHeaderValue method from GrpcProtocolHelpers
src/Grpc.Net.Client/Internal/GrpcProtocolHelpers.cs	Removed GetHeaderValue method and updated references
src/Grpc.Net.Common/Compression/DeflateCompressionProvider.cs	Added leaveOpen parameter to prevent stream disposal
test/Grpc.Net.Client.Web.Tests/GrpcWebResponseStreamTests.cs	Added test for compressed trailers functionality