feat: allow file:// protocol prefix in absolute paths

[alexander-akait]

Recreates #210 by @laymonage, rebased on main, with the original commit kept as a co-author. Fixes #209.

Summary

import.meta.resolve() returns a file:// URL, so passing one to an option that takes an absolute path failed validation. The absolutePath keyword accepts an optional file:// prefix now:

- schema === /^(?:[A-Za-z]:(\\|\/)|\\\\|\/)/.test(data);
+ schema === /^(?:file:\/\/)?(?:[A-Za-z]:(\\|\/)|\\\\|\/)/.test(data);

What kind of change does this PR introduce?

A feature, plus test coverage for it.

Did you add tests for your changes?

Yes. Beyond the two cases from the original PR, every option of webpack's own schema that takes an absolute path is covered - the point being to verify the support against a real consumer rather than the fixture schema alone.

Walking webpack/schemas/WebpackOptions.json from the root and resolving $refs gives 34 reachable options that use absolutePath: true, and each one now has a test that a file:// URL validates for it:

Area	Options
Top level	context, recordsPath, recordsInputPath, recordsOutputPath
output	output.path
cache	cacheDirectory, cacheLocation, immutablePaths, managedPaths
snapshot	immutablePaths, managedPaths, unmanagedPaths
module	noParse and noParse[]; for both rules and defaultRules: test, include, exclude, issuer, resource, realResource, resolve.restrictions
resolve	resolve.restrictions, resolveLoader.restrictions
experiments	buildHttp.cacheLocation, buildHttp.lockfileLocation
Other	dotenv.dir, stats.context

All 34 fail without the change to absolutePath.js and pass with it, so they are covering the behaviour rather than passing by accident.

Does this PR introduce a breaking change?

One small behaviour change, covered by its own tests: an option that takes a relative path (output.filename, output.sourceMapFilename, output.assetModuleFilename and friends, which use absolutePath: false) used to accept a file:// value, because it did not look absolute. Such a value is recognised as absolute now and is rejected for those options, which is what they mean. The changeset is a minor and calls it out.

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

Nothing - as in the original PR, there are no docs for the absolute path keyword.

🤖 Generated with Claude Code

https://claude.ai/code/session_013wqLmVHkAGBsWQgXknEQCK

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.78%. Comparing base (fc918a9) to head (863de9e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #222   +/-   ##
=======================================
  Coverage   95.78%   95.78%           
=======================================
  Files           9        9           
  Lines         949      949           
  Branches      386      387    +1     
=======================================
  Hits          909      909           
  Misses         36       36           
  Partials        4        4           

Flag	Coverage Δ
integration	95.78% <100.00%> (ø)

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