Refactor/api client error handling

[lewiszlw]

Background

Based on the corrected test reports, it was confirmed that the API returns {} for successful sends, and uses specific integer codes for business failures.

Refactoring Content

• Unified the API request pipeline by introducing a generic
  equest method to consolidate the HTTP execution flow, removing redundancy in post_json and post_form.
• Improved error handling to be more explicit and actionable.

Error Message Improvements

et: -2 $\rightarrow$ Invalid context token

et: -3 $\rightarrow$ User ID mismatch or not found

• errcode: -14 $\rightarrow$ Session expired

Verification

• cargo fmt --check ✅
• cargo check ✅
• cargo test ✅ (All 26 tests passed)