Add metadata support for HTTP.URI

[ViralBShah]

Summary

• Adds FileIO.metadata(uri::HTTP.URI) to the HTTP extension, matching the existing FileIO.load(uri::HTTP.URI) pattern
• Fixes metadata fails on HTTP.URI #402

Test plan

• metadata(HTTP.URI("https://...some-image-url...")) returns metadata instead of erroring

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.31%. Comparing base (4e544bd) to head (53b6096).
⚠️ Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
ext/HTTPExt.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #433      +/-   ##
==========================================
- Coverage   91.44%   91.31%   -0.13%     
==========================================
  Files          11       11              
  Lines         713      714       +1     
==========================================
  Hits          652      652              
- Misses         61       62       +1     

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