function_storage_cf_do: switch from http.client to httpx.Client

The previous implementation shared a single http.client.HTTPConnection
across threads, whose state machine (IDLE -> REQ_STARTED -> REQ_SENT)
corrupts under concurrent use. Symptom from the HTTP fixture worker
running concurrent producer turns: ResponseNotReady: Idle raised from
getresponse() because another thread reset the conn between request()
and getresponse().

httpx.Client is thread-safe by construction — its connection pool
serialises per-connection access — so the threading.local workaround
disappears, along with manual request/getresponse/read/json.loads
plumbing. Connect-level retries are delegated to HTTPTransport;
status- and transport-level retries (5xx, RemoteProtocolError,
non-JSON response bodies) stay in _post for control over the
backoff policy.

httpx is declared as a direct dependency now that it's load-bearing
in core code (it was already pulled in transitively for vgi/client/).

Tests rewritten on top of httpx.MockTransport. New regression
coverage: concurrent-thread safety, 5xx retry, retry exhaustion,
transport-error retry, non-JSON response retry, 4xx no-retry,
close() releases the client.

Author: rustyconover

pyproject.toml

@@ -11,6 +11,7 @@ dependencies = [
     "platformdirs",
     "vgi-rpc @ file:///Users/rusty/Development/vgi-rpc",
     "duckdb>=1.5.0",
+    "httpx>=0.24",
 ]
 
 [project.optional-dependencies]