fix: avoid impersonation for captcha API JSON requests

[zy6p]

Summary

• remove curl_cffi browser impersonation when calling captcha API JSON endpoints
• keep the existing AsyncSession client, but send plain JSON POSTs for createTask / getTaskResult
• apply the fix to both admin score-test and runtime FlowClient captcha token fetching

Why

curl_cffi with impersonate="chrome110"/"chrome120" can cause FastAPI/Uvicorn services to receive an empty request body for JSON POSTs.

That breaks self-hosted YesCaptcha-compatible services such as OhMyCaptcha with:

{"detail":[{"type":"missing","loc":["body"],"msg":"Field required","input":null}]}

Reproduction

Using curl_cffi==0.7.3 against a FastAPI/Uvicorn OhMyCaptcha service:

• AsyncSession().post(..., json=payload) => works
• AsyncSession().post(..., json=payload, impersonate="chrome110") => 422 / missing body
• AsyncSession().post(..., json=payload, impersonate="chrome120") => 422 / missing body

Validation

I validated the patched image locally against a live OhMyCaptcha instance:

• src.api.admin._solve_recaptcha_with_api_service(...) returned a token successfully
• FlowClient._get_api_captcha_token(...) returned a token successfully

Both paths now reach /createTask and /getTaskResult with normal 200 responses instead of 422 missing body.