Preserve request params and validate upload files in HTTPXAPITarget

[biefan]

Summary

• preserve params for non-GET HTTPXAPITarget requests
• preserve params for file-upload requests as well as JSON/form requests
• raise a clear FileNotFoundError when an explicitly provided upload path does not exist
• add regression coverage for query-parameter handling and explicit missing upload files

Problem

HTTPXAPITarget exposes a params constructor argument, but send_prompt_async() currently drops it outside the GET/HEAD path. That affects both standard API calls and file-upload requests.

The same method also accepts an explicit file_path, and its docstring says a missing file should raise FileNotFoundError. In practice, if that path does not exist, the upload branch is silently skipped and the request falls back to a normal non-upload request.

Examples that currently behave incorrectly:

• POST /endpoint?alpha=1 with json_data=...
• POST /upload?alpha=1 with file_path=...
• an explicit file_path pointing to a missing file

This makes valid API patterns impossible to express and hides configuration mistakes that should fail fast.

Testing

• .venv/bin/pytest tests/unit/target/test_http_api_target.py -q