Adds focused tests verifying that the static-asset layer serves CSS
files when the browser percent-encodes special characters in the path:
* `tests/serve-static.test.ts` — three cases against `tryServeStatic`:
the cache fast path with an encoded URL, the slow path with an
encoded URL, and the literal-character pathname that the request
pipeline produces after `normalizePathnameForRouteMatchStrict` runs.
* `tests/pages-css-url-encoding.test.ts` — full build-and-serve test
for the Pages Router that imports `my@style.css` and `with space.css`
from `pages/index.tsx`, then asserts every emitted stylesheet link
resolves to a 200 after `encodeURI` is applied (matching browser
behaviour).
All three tests pass against the current implementation: `tryServeStatic`
decodes percent-encoded pathnames before cache lookup, and the cache
indexes files by their literal on-disk names. These tests document the
contract and guard against future regressions.
Refs cloudflare/vinext#1472
James Anderson