test(content-blog): regression-test trailingSlash on feed URLs (AI-assisted)

[LeSingh1]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

Adds an explicit regression test for the trailing-slash behavior of blog feed URLs. The existing feed.test.ts > with trailing slash snapshot already captures the URLs, but a regression in feed.ts would just nudge the snapshot and could pass review unnoticed. This adds a programmatic assertion that fails loudly with a descriptive message.

What this PR does

In packages/docusaurus-plugin-content-blog/src/__tests__/feed.test.ts, inside the existing describe.each(['atom', 'rss', 'json']) block, adds one new test that:

• builds feeds with trailingSlash: true against the existing __fixtures__/website fixture,
• extracts URLs only from feed-structural positions (<id>, <link>, <guid>, JSON id / url / home_page_url / feed_url) because CDATA bodies and content_html may legitimately contain author-written non-slashed links,
• asserts every blog URL ends with /,
• wraps assertions in try / finally with fsMock.mockClear() so a failure in one parameterized variant doesn't leak fsMock state into the next.

No production code is changed.

Test plan

• yarn test packages/docusaurus-plugin-content-blog/src/__tests__/feed.test.ts — 24/24 pass.
• yarn tsc --noEmit for the package — clean.
• yarn lint:js — no new warnings.
• Mutation check: removed applyTrailingSlash from feed.ts:130 and confirmed all 3 new parameterized variants fail with the descriptive message atom/rss/json feed URL should end with "/"; restored after.

Related issue

Adds explicit regression coverage for #7656.

The behavior is already correct on current main, so this PR does not change production code. It makes the feed URL trailing-slash behavior fail loudly if it regresses again.

Notes

AI-assisted, per AGENTS.md.

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