fix(modelscope): validate ranged responses

[NelZyhh]

Related Issue

• dfget downloads tokenizer.json with same file size as origin, but content is corrupted and MD5 is different #1932

Summary

• add strict validation for ranged responses in the ModelScope backend
• reject ranged downloads unless the source returns 206 Partial Content
• verify Content-Range exists and matches the requested byte interval
• return backend failure instead of silently writing incorrect piece content
• add a regression test for the 200 OK + full body case on range requests

Background

When downloading modelscope:// files by pieces, the client previously treated any 2xx response as success.
If a ranged request unexpectedly returned a full body response, the client could still write the first N bytes to a non-zero offset.
This could produce files with the correct size but corrupted content.

A real reproduction showed:

• piece 0 content and offset 4194304 content were identical
• source file size matched, but file body was corrupted

Root Cause

The ModelScope backend propagated the Range request header, but it did not validate:

• response status is 206 Partial Content
• Content-Range header is present
• Content-Range start/end matches the requested range

As a result, an invalid ranged response could pass through as a successful piece download.

Changes

• add validate_range_response() in dragonfly-client-backend/src/model_scope.rs
• require 206 Partial Content for ranged requests
• require valid Content-Range for ranged requests
• mark the backend response as failed when range validation fails
• update the existing range success test to include Content-Range
• add a regression test that rejects 200 OK + full body for a range request

Testing

• passed: cargo fmt --check
• added: range success test with valid Content-Range
• added: regression test for 200 OK + full body
• note: local cargo check --all --all-targets and cargo clippy --all-targets -- -D warnings were blocked earlier by a broken local Rust 1.88.0 toolchain; the toolchain has been repaired and the full checks are being rerun

[codecov]

Codecov Report

❌ Patch coverage is 60.13986% with 57 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.26%. Comparing base (a4a57d6) to head (775d1cf).

Files with missing lines	Patch %	Lines
dragonfly-client-backend/src/model_scope.rs	60.13%	57 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1933      +/-   ##
==========================================
+ Coverage   47.21%   47.26%   +0.05%     
==========================================
  Files          94       94              
  Lines       24214    24356     +142     
==========================================
+ Hits        11432    11512      +80     
- Misses      12782    12844      +62     

Files with missing lines	Coverage Δ
dragonfly-client-backend/src/model_scope.rs	76.54% <60.13%> (-5.81%)	⬇️

... and 1 file with indirect coverage changes

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