AIMVT-303: Implement HTTP client in ParallelHTTPClient class

[speriaswamy-amd]

Summary

• Adds ParallelHTTPClient (cvs/core/agent/http_client.py), an async httpx-based client providing API parity with ParallelSSHClient: run_command, health, shutdown, lazy shared connection reuse, and rebuild/destroy for host-set changes.
• Adds HostOutput dataclass and exception mapping (HTTPConnectionError/HTTPProtocolError) so unreachable vs. failed-request hosts can be distinguished the same way prune_unreachable_hosts distinguishes pssh exceptions.
• Supports stop_on_errors, host_args (per-host command substitution), and inline/file output modes, collecting file-mode output back into list[str] for a uniform response shape.

Stacked on #365 (AIMVT-302) — this PR's diff is limited to http_client.py and its tests; base branch will be retargeted to main once #365 merges.

Jira: AIMVT-303

Test plan

• cvs/core/agent/unittests/test_http_client.py covers run_command (inline/file/exit-code-only modes, host_args, stop_on_errors), health, shutdown, and exception classification, all against mocked HTTP transport