fix: allow colon paths on POSIX

[mikamikasuki]

Description

Fixes #23620

Since #22572, isFileLoadingAllowed rejects any path containing : while server.fs.strict is on. The check runs on every OS, but : is a valid character in file and directory names on Linux and macOS. This made imports of files such as src/route:name/index.ts (e.g. files generated by Laravel Wayfinder) fail with a 403.

This PR narrows the check:

• On Windows, : is still rejected, since it can't be part of a file name there (it's used for NTFS alternate data streams).
• On other platforms, : is allowed. The path with the stream part removed is also checked against server.fs.deny, so .env::$DATA is checked as .env and is still denied.

Trade-off: on an NTFS volume mounted on Linux or macOS, streams of a file that is allowed and not denied become readable again, as they were before #22572.

Added tests for:

• <a path containing a colon is served on POSIX>
• <a stream-style path of a denied file is still denied>
• <colon paths are still rejected on Windows, if covered>

[github-actions]

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[mikamikasuki]

Hi, I believe this was flagged by mistake.

I opened this PR manually from my own account, and I wrote the code change myself after running into the problem described in #23620 [add how you hit it or reproduced it, e.g. a project with colon paths on Linux/macOS]. I understand the project doesn't accept PRs created by automated agents, and this isn't one.

I also think the change is a substantive fix rather than a drive-by edit:

• Since fix: reject windows alternate paths #22572, isFileLoadingAllowed rejects any path containing : on every OS, although : is a valid file name character on Linux and macOS. This breaks real projects (e.g. files generated by Laravel Wayfinder).
• The PR keeps rejecting : on Windows, allows it elsewhere, and still checks the path with the stream part removed against server.fs.deny, so . env::$DATA is still denied.
• Tests: [list the real test cases you added].

For transparency: The code and tests are my own work, and I'm happy to explain any part of the implementation or adjust it based on review.

Please let me know if you need anything else from me to review this.

[2010YOUY01]

• Tests: [list the real test cases you added].

🤔