pgconn: add configurable max message body length

[carter-ya]

Summary

• Add pgconn.Config.MaxBodyLen and apply it to pgproto3.Frontend during ConnectConfig and Construct.
• Add regression coverage for configured body length limits.

Rationale

pgproto3.Frontend already supports SetMaxBodyLen, but pgconn users currently need to reach through to the protocol frontend to configure it. Exposing the setting on pgconn.Config makes it possible to reject unreasonable message bodies before chunkReader allocates the buffer.

The default remains 0, so no maximum is enforced unless callers opt in. This preserves the existing behavior while allowing applications to configure a defensive limit.

Testing

• go test ./pgproto3 ./pgconn -run 'TestFrontendReceiveExceededMaxBodyLen|TestBackendReceiveExceededMaxBodyLen|TestConnectConfigMaxBodyLen|TestConfigCopyReturnsEqualConfig'

[jackc]

Is MaxBodyLen the right name for this at the pgconn.Config layer? It has a clear meaning at the pgproto3.Frontend layer. But at the connection / config layer I'm not sure I would immediately understand what it meant. 🤷