Fix webpack loader cache key for resource queries (#19723)

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## Summary

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your change solve the problem?

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`@tailwindcss/webpack` currently uses `this.resourcePath` as the cache
key, which ignores the resource query. When the same CSS file is
imported multiple times with different `resourceQuery` values, all of
those imports share a single `CacheEntry`. That means the utilities
discovered for one entry can leak into the CSS output for another entry.

This PR changes the cache key to use `this.resource` (path + query)
instead, while still using `this.resourcePath` for all filesystem work.

## Test plan

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- `pnpm test:integrations -- webpack/loader.test.ts`  
  - Confirms all existing webpack loader integration tests pass.  
- Confirms the new `@tailwindcss/webpack loader isolates cache by
resource including query` test passes, verifying that two entries
importing the same CSS file with different queries produce isolated
outputs (`dist/a.css` only contains `only-a` / `--color-red-500`, and
`dist/b.css` only contains `only-b` / `--color-blue-500`).

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Co-authored-by: Robin Malfait <malfait.robin@gmail.com>