Fix: validate byte count in file_message.py decode handlers

[SunandM]

Adds bounds checking to three decode handlers in pymodbus/pdu/file_message.py,
consistent with the fixes applied in #2974 and #2977.

Changes:

• ReadFileRecordRequest.decode: validate byte_count against len(data) before iterating
• WriteFileRecordRequest.decode: validate byte_count against len(data) before looping
• ReadFifoQueueResponse.decode: validate count against len(data) before iterating

Previously, an oversized byte_count field caused struct.unpack() to raise
struct.error, which escaped the ModbusException/ValueError/IndexError
handler in DecodePDU.decode(). The fix raises ModbusException instead,
matching the behaviour added in #2974 and #2977.

Related to security advisory GHSA-xxxx (private).

[janiversen]

LGTM, but you need to add tests:

The new "if" statements are not covered in the test harness and we are proud to have 100% coverage.

easiest way to test locally:

run "purest --cov" in the main directory.

[SunandM]

Hi @janiversen, I have added three tests covering the new validation branches (test_read_file_record_request_decode_invalid_byte_count, test_write_file_record_request_decode_invalid_byte_count, test_read_fifo_queue_response_decode_invalid_count). All 16 CI checks are now passing with 100% coverage. Could you take another look when you get a chance? Thanks!

[janiversen]

LGTM, thanks

[SunandM]

Thank you so much for your assistance :)