fix: increase HTTP/2 connection window default from 64KB to 1MB (#6238)

[waterWang]

Fixes #6238

Description

The default HTTP/2 connection window size is -1, which means Netty's default stream initial window size (64KB) is used for the connection window. For large payloads (e.g., 10MB+ gRPC messages), this requires hundreds of WINDOW_UPDATE round-trips before the application receives the full body, causing 25s+ latency vs ~1s with a properly sized window.

Changes

• HttpServerOptions.java: Changed DEFAULT_HTTP2_CONNECTION_WINDOW_SIZE from -1 to 1024 * 1024 (1MB)
• HttpClientOptions.java: Same change for consistency

Why 1MB?

• Matches Netty's default HTTP/2 connection window size used by the legacy gRPC-Java/Netty stack
• Large enough to avoid excessive WINDOW_UPDATE round-trips for typical large messages (10MB+)
• Small enough to not significantly increase memory pressure (the window is a budget, not an allocation)
• Users who need a different value can still configure it via setConnectionWindowSize()

Verification

• The condition connectionWindowSize > 0 in HttpServerConnectionHandler.java already handles positive values correctly
• With the new default of 1MB, setWindowSize() will be called with a reasonable value
• No behavioral change for users who explicitly configure setConnectionWindowSize()

[vietj]

the javadoc should be updated accordingly