Tests: Add unit test coverage for WP_Http_Encoding

[huzaifaalmesbah]

Description

Adds a test class for WP_Http_Encoding, which had no tests. The tests cover compress() and its compression level, decompress() for raw deflate, zlib and gzip data and for content it must return unchanged, compatible_gzinflate() with the header checksum, file name and comment gzip header fields and with invalid data, the Accept-Encoding value built by accept_encoding() for each request argument that disables compression, the wp_http_accept_encoding filter arguments and return value, content_encoding(), should_decode() for array, string and non-string headers, and is_available().

Compressed fixtures are built in the test and every expectation is the original literal content.

This is a test-only change; no production code is modified.

Testing Instructions

1. Run the tests on single site:
   npm run test:php -- --filter 'Tests_HTTP_wpHttpEncoding'
2. Run them on multisite:
   npm run test:php -- --filter 'Tests_HTTP_wpHttpEncoding' -c tests/phpunit/multisite.xml
3. Run them in random order:
   npm run test:php -- --filter 'Tests_HTTP_wpHttpEncoding' --order-by=random
4. Run the whole group to confirm nothing else is affected:
   npm run test:php -- --group http
5. Run PHPCS on the test file:
   node ./tools/local-env/scripts/docker.js run --rm php ./vendor/bin/phpcs tests/phpunit/tests/http/wpHttpEncoding.php

Trac ticket: https://core.trac.wordpress.org/ticket/65819

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5.5
Used for: Brainstorming edge-case scenarios and drafting test boilerplate; test architecture, assertions, and local PHPUnit verification were written and validated by me.

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