fix(backend): use full error chain in stream read failures

[pmady]

Description

The map_err closures in get() across all three backends walk the error source chain into a chain string but then pass the original err to IOError::new() instead of chain. The full error context gets computed and thrown away.

Fixes http, hugging_face, and model_scope backends.

Motivation and Context

When a stream read fails mid-download, the IOError only contains the top-level message. Nested causes (DNS, TLS, connection reset, etc.) are lost. This makes debugging download failures harder than it needs to be.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 44.09%. Comparing base (f1ce94b) to head (24ba736).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
dragonfly-client-backend/src/http.rs	0.00%	1 Missing ⚠️
dragonfly-client-backend/src/hugging_face.rs	0.00%	1 Missing ⚠️
dragonfly-client-backend/src/model_scope.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #1861 +/- ##
==========================================
- Coverage 44.11% 44.09% -0.03% 
==========================================
Files 93 93 Lines 27564 27564 ==========================================
- Hits 12161 12154 -7 - Misses 15403 15410 +7 

Files with missing lines	Coverage Δ
dragonfly-client-backend/src/http.rs	96.09% <0.00%> (ø)
dragonfly-client-backend/src/hugging_face.rs	40.14% <0.00%> (ø)
dragonfly-client-backend/src/model_scope.rs	46.74% <0.00%> (ø)

... and 1 file with indirect coverage changes

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[gaius-qi]

LGTM