test(blog): add simple regression test for trailingSlash (AI-assisted)

[04AvinashSingh]

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

Fixes #7656.

This PR adds a simple string match regression check to the existing test suite to verify that trailingSlash: true correctly applies to blog feeds.

This explicitly addresses the previous maintainer feedback that PR #12022 was overly complex, achieving the exact same regression coverage in just 4 lines of code without requiring dynamic XML/JSON parsing.

(AI-assisted)

Test Plan

Ran pnpm test over the entire repository. The new assertion in packages/docusaurus-plugin-content-blog/src/__tests__/feed.test.ts fails loudly if applyTrailingSlash is ever accidentally removed from the feed generator logic, but passes cleanly with the current correct behavior.

Additionally, I fixed two hidden TypeScript compilation errors inside feed.test.ts (a broken DEFAULT_VCS_CONFIG import and an outdated I18nLocaleConfig mock) that were incorrectly being skipped by __tests__ exclusions in the TS config.

Test links

N/A - Unit test change only.

Related issues/PRs

Fixes #7656
Supersedes #12022

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