fix: include file names in CSS minification warnings

[pierreeurope]

Description

Fixes #15915

When CSS minification (via esbuild or lightningcss) produces warnings or errors, they now display the actual filename instead of generic placeholders like <stdin> or style.css. This makes it significantly easier to debug which CSS file has the issue.

Changes

• Added an optional filename parameter to the minifyCSS() function
• Passed it to esbuild's sourcefile option to provide context in warnings/errors
• Passed it to lightningcss's filename option (addressing the existing TODO comment)
• Updated finalizeCss() to accept and forward the filename parameter
• Updated all call sites to pass the actual filename when available:
  • For single-file CSS (e.g., ?inline imports): uses the actual file path
  • For aggregated CSS chunks: uses the chunk name or CSS asset name
  • For extracted CSS: uses the CSS bundle name

Before

warnings when minifying css:
▲ [WARNING] Unexpected "{" [css-syntax-error]

    <stdin>:484:1:
      484 │  {
          ╵  ^

After

warnings when minifying css:
▲ [WARNING] Unexpected "{" [css-syntax-error]

    path/to/actual/file.css:484:1:
      484 │  {
          ╵  ^

Related

• This is a fresh implementation inspired by the approach in feat(css): provide filename for css-minify error #16006, but simplified to avoid the complexity of line mapping for concatenated CSS
• The fix covers the most common use case (single-file minification warnings) while also providing better context for aggregated chunks
• All existing tests pass

Checklist

• Read the Contributing Guidelines
• Checked that there isn't already a PR that solves the problem
• Provided a description of what the PR solves
• Ran existing tests to ensure nothing breaks