perf: raise the default connection pool size from 5 to 100

[mogita]

Problem

The default transport capped each client at 5 concurrent connections per host (MaxConnsPerHost = 5). The API is served over HTTP/1.1, so a 6th concurrent request waits for a free connection, and that wait counts against the request timeout.

Solution

• Raise the WithMaxConnsPerHost default from 5 to 100. It also sets MaxIdleConnsPerHost, so up to 100 connections are kept warm for reuse. Idle connections still close after 55s.
• Set MaxIdleConns to 0. The cloned http.DefaultTransport caps idle connections at 100 across all hosts, which would otherwise override a larger WithMaxConnsPerHost.
• 100 matches stream-py (#299).
• Apps that relied on the old limit to throttle traffic can pass WithMaxConnsPerHost(5).

How to verify

1. go test -short -run 'TestClientDefaultTransportConfig|TestClientInfoLogOnConstruction' -v .
2. Both pass and assert MaxConnsPerHost and MaxIdleConnsPerHost at 100, MaxIdleConns at 0, and the client.initialized log line at 100.