fix: strip BOM before processing with postcss

[alexander-akait]

Summary

Fixes #1678. postcss stripped a leading BOM until 8.5.24 and preserves it since, so a BOM added by an earlier loader (sass-loader with charset: true, its production default) survived into the module and landed mid-file once stylesheets were concatenated, silently dropping the rule after it.

This builds on @ShGKme's #1679 and credits that analysis; it differs in one respect. content is not always a string — when a previous loader hands over its AST via meta.ast, content is a postcss Root, and charCodeAt on it throws TypeError: content.charCodeAt is not a function (8 existing tests fail that way). So the strip is applied to both inputs postcss accepts: the string, and the Root, where the BOM lives on source.input.hasBOM rather than in the tree. The sass-loader → postcss-loader → css-loader chain takes that second path.

What kind of change does this PR introduce?

fix

Did you add tests for your changes?

Yes — two cases in test/loader.test.js with fixtures in test/fixtures/modules/issue-1678/, one per input path. Both were confirmed to fail without their half of the fix, and the whole suite passes on postcss 8.5.5 (the lockfile version) and on 8.5.26, so they do not depend on which patch is installed.

Does this PR introduce a breaking change?

No — it restores the output postcss produced before 8.5.24.

If relevant, what needs to be documented once your changes are merged or what have you already documented?

n/a

Use of AI

Yes — Claude Code wrote the tests first and confirmed each fails without the corresponding branch of the fix, found the Root case by running the existing suite against postcss 8.5.26, and verified the suite on both postcss versions. Reviewed and edited by me before pushing.

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.28%. Comparing base (488cdb1) to head (ad75b64).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1684      +/-   ##
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+ Coverage   96.25%   96.28%   +0.02%     
==========================================
  Files          10       10              
  Lines        1201     1210       +9     
  Branches      463      468       +5     
==========================================
+ Hits         1156     1165       +9     
  Misses         40       40              
  Partials        5        5              

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