fix: reduce upload_security false positives

[google-labs-jules]

What

Reduces false positives in upload_security.py regarding static file serving and allowlist checks.

Why

• Statically serving internal files using res.sendFile(__dirname ...) shouldn't trigger the serve-no-nosniff alert since it is not serving user-generated uploads.
• Checking mimetypes against generic allowlists like validTypes and supportedMimeTypes falsely triggered upload-trusts-client-mime.

Risk

Low. Adds context awareness for static files explicitly mapped via __dirname or process.cwd(). If a file has mixed static and dynamic file serving, the new logic iterates and ensures that if any dynamic serve sink exists, the file is still correctly flagged. Also expanded the regex for allowlist variables. Added regression tests.

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PR created automatically by Jules for task 15566643719533193340 started by @raccioly

[google-labs-jules]

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