fix: invalidate chunk hashes on renderBuiltUrl change (fix #13996)

[hartz89]

Summary

Fixes #13996. When experimental.renderBuiltUrl changes, JS and CSS chunk hashes now invalidate properly.

• JS chunks: Uses Rollup's augmentChunkHash hook to include the stringified renderBuiltUrl function in hash computation
• CSS assets: Injects a hash of renderBuiltUrl into CSS content via finalizeCss (before the asset filename hash is computed), then strips it in generateBundle so it doesn't appear in shipped CSS

Known limitation: Hash computation uses Function.prototype.toString(), so only the function's source text is captured. If renderBuiltUrl closes over external state (e.g. process.env.CDN_URL) without referencing it in the function body, changing that value won't produce new filenames — the function body itself must change. This is noted in a code comment and matches the general contract of augmentChunkHash.

Testing

• Unit tests in build.spec.ts verifying that different renderBuiltUrl functions produce different output hashes for both JS chunks and CSS assets
• New playground/render-built-url integration test running two real builds with different renderBuiltUrl configs and asserting filenames differ for both JS chunks and CSS assets
• Dogfooded in a production app using renderBuiltUrl for CDN asset serving — confirmed all 19 JS chunks and 3 CSS assets received new hashes after modifying the function

[sapphi-red]

Thanks for the PR.
I've made a PR that resolves the issue without the limitation in #22888, #22984.