fix(extract-comments): decode Buffer sources before merging

[tlaurie-cl]

Summary

Fixes #730.
Merging extracted comments into a file another plugin emitted calls .split("\n\n") on its source(), which is string | Buffer. If the source is a Buffer, it crashes the build with prevSource.source.split is not a function.

The fix checks whether source() actually returned a string or Buffer, and decodes it with toString("utf8") when it's a Buffer. The /** @type {string} */ assertion is removed.

This crash first appears in 5.9.0, but the /** @type {string} */ assertion is older: it was added in terser-webpack-plugin #381, when the merge only ever saw files this plugin itself had written, which were always strings. #716 then started merging into comment files that other plugins had already emitted, causing Buffers to newly reach .split(). webpack 5.110.3 depends on this plugin, so a project can hit the crash without depending on it directly.

What kind of change does this PR introduce?

A fix.

Did you add tests for your changes?

Yes, one new test: should work with the existing licenses file, when it is a Buffer.
The new test fails on main with the TypeError from the issue and passes with this change.

Does this PR introduce a breaking change?

No.

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

None needed.

Use of AI

Cursor's Auto Intelligence mode was used with supervision to debug the issue, trace the regression, build the reproduction and fix branches, and draft the bug and PR. All submission output and code has been reviewed and cleaned up by myself.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: tlaurie-cl / name: Thomas Laurie (47a31e2)

[changeset-bot]

🦋 Changeset detected

Latest commit: 47a31e2

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
minimizer-webpack-plugin	Patch

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[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.22%. Comparing base (649b48d) to head (47a31e2).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/index.js	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #731      +/-   ##
==========================================
- Coverage   95.27%   95.22%   -0.06%     
==========================================
  Files           4        4              
  Lines        1354     1360       +6     
  Branches      503      501       -2     
==========================================
+ Hits         1290     1295       +5     
- Misses         56       57       +1     
  Partials        8        8              

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