feat(fetch): support file URLs via opt-in dispatcher interceptor

[mcollina]

Summary

• add a new interceptors.file() dispatcher interceptor for handling file: URLs
• keep policy app-defined and deny-by-default via allow() callback
• do not rely on Node process permissions
• wire fetch() file scheme path to dispatcher so custom dispatchers/interceptors can handle it

Details

• New interceptor: lib/interceptor/file.js
• Exported via interceptors.file in index.js
• Added types in types/interceptors.d.ts
• Added docs in docs/docs/api/Dispatcher.md

Tests

• test/interceptors/file.js
  • serves allowed files
  • blocks disallowed files
  • passes through non-file requests
• test/fetch/file-url-interceptor.js
  • default fetch(file://...) rejects
  • fetch(file://...) works with custom file interceptor
  • disallowed policy still rejects

[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.24339% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.88%. Comparing base (2885361) to head (bef3811).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/interceptor/file.js	85.63%	26 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4907      +/-   ##
==========================================
- Coverage   92.90%   92.88%   -0.02%     
==========================================
  Files         112      113       +1     
  Lines       35638    35821     +183     
==========================================
+ Hits        33109    33272     +163     
- Misses       2529     2549      +20     

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[KhafraDev]

This comment comes from the spec and should be kept.

[mcollina]

What do you think @KhafraDev, will this be a better model?

[KhafraDev]

Yes, I have no problems with it. Is there an interest in this again, I saw my old PR getting activity too?

[mcollina]

Yes, I have no problems with it. Is there an interest in this again, I saw my old PR getting activity too?

it was marked as semver-major, and I took a look at it again before cutting undici 8.