Tests: Add unit test coverage for search feed links

[rejaulalomkhan]

Description

Adds a test class for get_search_feed_link() and get_search_comments_feed_link(), which had no tests.

The tests cover:

• Plain and pretty permalinks, with the default feed and an explicit atom feed, and a query with spaces.
• The comments feed: feed=comments-<type> with plain permalinks and ?withcomments=1 with pretty permalinks.
• The default_feed filter being used when no feed type is passed.
• The search_feed_link filter: its arguments ($link, $feed, 'posts') and filtered return value for the posts feed, and for the comments feed the two calls it makes (first 'posts' for the link it builds on, then 'comments') with the filtered return value.

Each behaviour was checked against a mutated source (default feed ignored, missing trailing slash, wrong search type, missing comments- prefix, missing withcomments, filter result discarded); a test fails for every mutation.

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

Testing Instructions

1. npm run test:php -- --filter Tests_Link_GetSearchFeedLink
2. npm run test:php -- -c tests/phpunit/multisite.xml --filter Tests_Link_GetSearchFeedLink
3. npm run test:php -- --group link,feed

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: Drafting the tests and running the checks. I reviewed the changes, ran the tests locally, and take responsibility for them.

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