Fix HTTP caching compliance by adding Vary headers for compressible content

[mlibbey]

The compress plugin was only adding Vary: Accept-Encoding headers when content was actually going to be compressed, not when content could be compressed. This can cause downstream caches to never get the compressed version in cache.

Now adds Vary: Accept-Encoding headers for all compressible content regardless of whether compression is applied, ensuring proper HTTP cache behavior.

[mlibbey]

@JakeChampion because of big recent changes to compress.

[bneradt]

In case it's helpful:
https://ci.trafficserver.apache.org/job/Github_Builds/job/centos/5791/console

../../../plugins/compress/compress.cc: In function 'void Compress::{anonymous}::handle_range_request(TSMBuffer, TSMLoc, const Compress::HostConfiguration*)':
../../../plugins/compress/compress.cc:100:35: error: passing 'const Compress::HostConfiguration' as 'this' argument discards qualifiers [-fpermissive]
  100 |     switch (hc->range_request_ctl()) {
      |                                   ^
In file included from ../../../plugins/compress/compress_common.h:41,
                 from ../../../plugins/compress/compress.cc:36:
../../../plugins/compress/configuration.h:94:3: note:   in call to 'Compress::RangeRequestCtrl Compress::HostConfiguration::range_request_ctl()'
   94 |   range_request_ctl()
      |   ^~~~~~~~~~~~~~~~~

[mlibbey]

Sorry for the initial build failures -- it was building locally :( (The Rocky crash looks to be in a cache subsystem regression test).
Think it really ready for review.

[cmcfarlen]

Seems good. I looked for, but did didn't notice any, missing API handle cleanup.

[Copilot]

Pull request overview

This PR fixes HTTP caching compliance by ensuring the Vary: Accept-Encoding header is added for all compressible content, not just when compression is actually applied. Previously, the compress plugin only added this header when actively compressing responses, which could cause downstream caches to incorrectly serve uncompressed versions to clients that accept compression.

Key changes:

• Refactored compression eligibility logic into separate functions (is_content_compressible and client_accepts_compression) for clearer separation of concerns
• Added dedicated function to set Vary header for all compressible content regardless of whether compression is applied
• Introduced orchestration function that handles both compression transformation and Vary header addition

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
plugins/compress/compress.cc	Refactored transformable function into is_content_compressible and client_accepts_compression; added add_vary_header_for_compressible_content and handle_compression_and_vary functions to ensure Vary header is added for all compressible content; removed vary_header check from compression transform initialization
tests/gold_tests/pluginTest/compress/compress.test.py	Added test cases to verify Vary header is present for compressible content without Accept-Encoding header and with unsupported Accept-Encoding values
tests/gold_tests/pluginTest/compress/compress.gold	Updated expectations to show Vary header appearing before Content-Encoding in header order, and added Vary header expectations for compressible but non-compressed responses
tests/gold_tests/pluginTest/compress/compress_userver.gold	Added expectations for new test cases testing vary header behavior with non-standard accept-encoding values
tests/gold_tests/pluginTest/compress/compress_vary.gold	New gold file containing expected Vary header output for new test cases

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[cmcfarlen]

Added to 10.1.1 milestone via #12764