Modbus: Move the TCP framing constants to the shared symbols header

mbpaHeaderSize was declared in both TCP transports with different types,
a static const qint8 in the server and a non-static int member in the
client; maxBytesModbusADU only in the server. Both describe the wire
format rather than either endpoint, so declare them once in
qmodbus_symbols_p.h. Names and values are unchanged, so no use site
moves and no behaviour changes.

First in the series, so the framing fixes that follow share one
definition.

Patch created with the help of Claude Opus 5, based on a deep-scan that
got concluded on this repository. Verified and corrected by myself.

Task-number: QTBUG-149968
Task-number: QTBUG-149969
Pick-to: 6.12 6.12.0 6.11 6.11.3 6.10 6.8
Change-Id: I0b06219fef039299076c2ce746ba44fe80e41744
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>