enhancement: Forward 5XX 4XX from source to client

[entrop-tankos]

Description

This change improves proxy-mode error propagation by forwarding backend HTTP error responses more transparently to the client.

Previously, when a backend/origin returned a non-success HTTP status, dfdaemon could preserve the status code in some cases, but the response body was lost on the proxy path. As a result, clients only saw the status code and could not access the backend-provided error message.

With this patch, when dfdaemon is used as an HTTP proxy and the backend returns a non-success response before normal body streaming starts, dfdaemon now forwards:

the backend HTTP status code,
the backend response headers,
the backend response body, if present.

This applies to proxy-mode error handling and preserves important backend-side context for callers.
Related Issue

N/A
Motivation and Context

Some origins return meaningful error details in the response body together with 4xx/5xx statuses. Typical examples include:

quota exceeded,
request rate limiting,
temporary bans,
access restrictions,
backend-specific retry hints.

That information is important for clients building their own retry and backoff logic on top of Dragonfly.

Without the response body, the caller only sees a generic HTTP error code and loses the backend-specific reason, which makes it much harder to distinguish between retryable and non-retryable failures.

This patch fixes that behavior by preserving and forwarding the backend error body to the client when available.

[codecov]

Codecov Report

❌ Patch coverage is 10.07752% with 116 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.44%. Comparing base (72fd774) to head (249e78b).

Files with missing lines	Patch %	Lines
dragonfly-client/src/proxy/mod.rs	0.00%	80 Missing ⚠️
dragonfly-client-backend/src/http.rs	38.88%	11 Missing ⚠️
dragonfly-client-backend/src/object_storage.rs	45.45%	6 Missing ⚠️
dragonfly-client-backend/src/hdfs.rs	0.00%	5 Missing ⚠️
dragonfly-client-backend/src/hugging_face.rs	0.00%	4 Missing ⚠️
dragonfly-client-backend/src/model_scope.rs	0.00%	4 Missing ⚠️
dragonfly-client-backend/src/opencsg.rs	0.00%	4 Missing ⚠️
dragonfly-client/src/proxy/task.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2031      +/-   ##
==========================================
- Coverage   54.64%   54.44%   -0.20%     
==========================================
  Files         101      101              
  Lines       27460    27570     +110     
==========================================
+ Hits        15005    15010       +5     
- Misses      12455    12560     +105     

Files with missing lines	Coverage Δ
dragonfly-client-backend/src/lib.rs	92.85% <ø> (ø)
dragonfly-client-core/src/error/errors.rs	87.50% <ø> (ø)
dragonfly-client-util/src/http/mod.rs	99.43% <100.00%> (+<0.01%)	⬆️
dragonfly-client/src/proxy/header.rs	85.19% <ø> (ø)
dragonfly-client/src/resource/persistent_task.rs	0.00% <ø> (ø)
dragonfly-client/src/resource/piece.rs	36.23% <ø> (ø)
dragonfly-client/src/resource/task.rs	3.04% <ø> (ø)
dragonfly-client/src/proxy/task.rs	0.00% <0.00%> (ø)
dragonfly-client-backend/src/hugging_face.rs	77.08% <0.00%> (-0.75%)	⬇️
dragonfly-client-backend/src/model_scope.rs	84.24% <0.00%> (-0.64%)	⬇️
... and 5 more

... and 1 file with indirect coverage changes

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