fix: sync local flow controller initial window with SETTINGS value

[waterWang]

Problem

The Triple HTTP/2 client advertises a stream window size via SETTINGS (SETTINGS_INITIAL_WINDOW_SIZE) but the local Http2LocalFlowController retains the Netty default of 65,535 bytes. This mismatch causes two failure modes:

1. 8 MiB window: The Java client tells the PHP server "send up to 8 MiB", the server sends 92 KiB of data, but the local flow controller rejects it with "Flow control window exceeded for stream".
2. 64 KiB window: The PHP server exhausts the advertised window and waits for a WINDOW_UPDATE, but the client never sends one because the gRPC message cannot be assembled from partial data.

Root Cause

In createHttp2ClientConnection() and createHttp2ServerConnection(), a custom TripleHttp2LocalFlowController is created and attached to the connection, but its initial window size is never updated from the default 65,535 to match the value broadcast in the SETTINGS frame. The local flow controller enforces a smaller window than what was advertised to the peer.

Fix

After attaching the flow controllers, call connection.local().flowController().initialWindowSize(initialWindowSize) to synchronize the local enforcement window with the advertised SETTINGS value. This is applied in both createHttp2ClientConnection() and createHttp2ServerConnection().

Fixes #16427