adding HttpClient tests (#785)

Add comprehensive `HttpClient` test suite. Covers:

- **HTTP verbs** -- GET, POST, PUT, DELETE, HEAD dispatch correctly
- **GET/POST/PUT/Upload/Download** -- payload round-trips (IoBuffer, CbObject, CompositeBuffer), content types, large payloads, file-spill downloads
- **Status codes** -- 2xx/4xx/5xx classification, exact code matching
- **Response API** -- IsSuccess, AsText, AsObject, ToText, ErrorMessage, ThrowError
- **Error handling** -- connection refused, request timeout, nonexistent endpoints
- **Session management** -- default ID, SetSessionId, reset to zero
- **Authentication** -- token provider, expired tokens, bearer verification
- **Content type detection** -- text, JSON, binary, CbObject
- **Request metadata** -- elapsed time, upload/download byte counts
- **Retry logic** -- retry after transient 503s, no-retry baseline
- **Latency measurement** -- MeasureLatency against live and unreachable servers
- **KeyValueMap** -- construction from pairs, string_views, initializer lists
- **Transport-level faults (GET)** -- connection reset/close before response, partial headers, truncated body, mid-body reset, stalled response timeout, retry after RST
- **Transport-level faults (POST)** -- server reset/close before consuming body, mid-body reset, early 503 without consuming upload, stalled upload timeout, retry with large body after transient failures

Also adds zenhttp-test to the xmake test runner (xmake test --run=http).

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