Mark HTTP/1 trailer headers as unsupported on Browser

[Copilot]

Description

HTTP/1 trailer headers are not implemented for Browser platform. Changed test attribute from ActiveIssue to SkipOnPlatform to reflect this is a permanent platform limitation, not a tracked bug.

Changes

• Test Attributes: Replaced [ActiveIssue("https://github.com/dotnet/runtime/issues/54156", TestPlatforms.Browser)] with [SkipOnPlatform(TestPlatforms.Browser, "HTTP/1 trailers are not supported by most major browsers")] on SocketsHttpHandler_Http1_TrailingHeaders_Test

This aligns with the pattern used for other Browser platform limitations (proxies, certificates, sockets). The skip message clarifies that this is a browser limitation affecting HTTP/1 trailers specifically, not a .NET implementation issue.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[wasm] Trailer headers not implemented</issue_title>
<issue_description>As covered by unit test SocketsHttpHandler_Http1_TrailingHeaders_Test and https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Trailer</issue_description>

<agent_instructions>Replace the ActiveIssue with platform not supported (or what is it called)</agent_instructions>

Comments on the Issue (you are @copilot in this section)

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Issue Details

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As covered by unit test SocketsHttpHandler_Http1_TrailingHeaders_Test and https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Trailer

Author:	pavelsavara
Assignees:	-
Labels:	arch-wasm, area-System.Net.Http
Milestone:	-

• Fixes [wasm] Trailer headers not implemented #54156

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[oroztocil]

@copilot See the comment #54156 (comment)

Do not try to implement the feature. Just replace the ActiveIssue attribute on the SocketsHttpHandler_Http1_TrailingHeaders_Test with a platform not supported for browser.

[Copilot]

Pull request overview

Updates System.Net.Http functional tests to reflect that HTTP/1 trailer headers are a Browser platform limitation rather than an active product issue.

Changes:

• Replaced ActiveIssue with SkipOnPlatform for the HTTP/1 trailing headers test on Browser.
• Updated the skip message to clarify that most major browsers don’t support HTTP/1 trailers.