fix(asset): preserve IPv6 brackets in server origin for ?url imports

[vaibhavmashal]

Problem:
When server.origin contains an IPv6 address, Vite percent-encodes the square brackets when generating URLs for ?url imports. Since the brackets are part of the IPv6 host, they should remain literal instead of being percent-escaped. As a result, browsers like Firefox correctly reject the generated value during fetch() because %5B::1%5D is an invalid hostname, leading to TypeError: Window.fetch: http://%5B::1%5D:5173/... is not a valid URL. This primarily breaks loading of external assets (e.g. WASM or web workers) when the server.origin is explicitly set to an IPv6 literal (e.g. http://[::1]:5173).

Solution:
The problem occurs in encodeURIPath where we use encodeURI(filePath). The standard ECMAScript encodeURI percent-encodes square brackets ([ and ]). Since we use this function to encode the final generated asset URLs, any valid IPv6 bracket characters in the host origin inadvertently become escaped.

The solution ensures we preserve literal brackets [ and ] but ONLY within the host part of the URL (i.e., http://[::1]). We do this by applying a regex to the encoded URI that only targets the <protocol>://<host> substring and safely un-encodes %5B and %5D back to [ and ], while leaving any encoded brackets in the pathname intact. This surgical approach maintains the percent-encoding for file paths that genuinely contain square brackets.

Changes Made:

• Modified encodeURIPath in packages/vite/src/node/utils.ts to replace %5B and %5D back into literal [ and ] characters exclusively within the host protocol portion of the URI string.
• Added tests in packages/vite/src/node/__tests__/utils.spec.ts for encodeURIPath to verify it correctly encodes file paths, while strictly ignoring brackets in the host component of the IPv6 address.

Testing:
To reproduce:

1. Initialize a minimal Vite project with an asset import assetUrl from './assets/vite.svg?url'.
2. Configure vite.config.js with server: { origin: 'http://[::1]:5173' }.
3. Try fetching assetUrl from the application in Firefox; observe TypeError due to invalid URL structure.

With this fix, the tests are fully green (pnpm test-unit utils), ensuring backwards compatibility while the origin correctly remains http://[::1]:5173 upon usage.

Fixes #23108

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