feat(config): add configurable max_retries for backend requests

[gaius-qi]

Description

This pull request introduces a configurable maximum retry count for backend HTTP requests, replacing the previous hardcoded value. The change allows users to specify how many times a failed backend request should be retried before giving up, improving flexibility and error handling. The new configuration is exposed in the Backend struct, with a sensible default, and is wired through the client creation logic.

Configuration and API changes:

• Added a max_retries field to the Backend struct in dragonfly-client-config/src/dfdaemon.rs, including serde support and a default value of 1. [1] [2] [3]
• Updated the HTTP backend constructor and struct in dragonfly-client-backend/src/http.rs to accept and store the max_retries parameter, and use it when building retry policies. [1] [2] [3] [4] [5]
• Updated BackendFactory in dragonfly-client-backend/src/lib.rs to pass the configured max_retries value when constructing HTTP/HTTPS backends. [1] [2]

Code cleanup:

• Removed the now-unused constant MAX_RETRY_TIMES from dragonfly-client-backend/src/lib.rs.
• Removed the MAX_RETRY_TIMES import from dragonfly-client-backend/src/http.rs.

Dependency and versioning:

• Bumped all crate versions from 1.3.7 to 1.3.8 in Cargo.toml to reflect the new feature. [1] [2]

Related Issue

Motivation and Context

Screenshots (if appropriate)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.56%. Comparing base (1f448d5) to head (99f7e29).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1817      +/-   ##
==========================================
+ Coverage   43.53%   43.56%   +0.02%     
==========================================
  Files          92       92              
  Lines       26917    26929      +12     
==========================================
+ Hits        11719    11731      +12     
  Misses      15198    15198              

Files with missing lines	Coverage Δ
dragonfly-client-backend/src/http.rs	96.09% <100.00%> (+0.02%)	⬆️
dragonfly-client-backend/src/lib.rs	96.33% <100.00%> (+0.02%)	⬆️
dragonfly-client-config/src/dfdaemon.rs	89.35% <100.00%> (+0.04%)	⬆️

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