fix(content-blog): fix wrong path variable in feed XSLT CSS file validation

[akshatsinha0]

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

In resolveXsltFilePaths inside packages/docusaurus-plugin-content-blog/src/feed.ts, there are two
sequential existence checks:

1. Line 202: checks if the XSLT file exists -- uses xsltAbsolutePath
2. Line 216: checks if the co-located CSS file exists -- also uses xsltAbsolutePath

The 2nd check should use cssAbsolutePath. Because it rechecks the XSLT path (which already passed
the first check), the CSS validation never fails ;even when the CSS file is genuinely missing. The
downstream fs.copy(cssAbsolutePath, ...) at line 252 then crashes with an unhelpful filesystem error
instead of the err message that was written for this exact scenario.

Test Plan

1.)the logic is straightforward.
2.)The function is called once by generateXsltFiles (line 242), which destructures {xsltAbsolutePath, cssAbsolutePath} and copies both files. With this fix, a missing CSS file is now caught at validation
time with the intended error message instead of failing silently until the copy step.

Test links

N/A — no UI change.

Related issues/PRs

None - analyis of file

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[slorber]

thanks 👍