HTTP2: Respect SETTINGS_ENABLE_PUSH

SETTINGS_ENABLE_PUSH (0x2):
This setting can be used to disable server push (Section 8.2).
- An endpoint MUST NOT send a PUSH_PROMISE frame if it receives this parameter set to a value of 0.
- (client) An endpoint that has both set this parameter to 0 and had it acknowledged MUST treat the receipt of a PUSH_PROMISE frame as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

The initial value is 1, which indicates that server push is permitted. Any value other than 0 or 1 MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

Start by tracing the HTTP/2 SETTINGS handling and PUSH_PROMISE processing in the repository, then compare the behavior with the requirements quoted in the issue. Done means values other than 0 or 1 produce PROTOCOL_ERROR, push is disabled after an acknowledged 0, and the initial value permits server push.

Written by the indexing model from the issue text.