Validate sequence number in nextReject()

[chrjohn]

Fixes #1235

[chrjohn]

@copilot please create a unit test that ensures that sequence numbers are always validated when receiving a Reject message. Test different scenarios (too high, too low) and check that the expected sequence number is incremented when it is the expected sequence number.

[Copilot]

@copilot please create a unit test that ensures that sequence numbers are always validated when receiving a Reject message. Test different scenarios (too high, too low) and check that the expected seq...

Added and pushed a focused unit test update in SessionTest to explicitly cover Reject MsgSeqNum scenarios in this order: too high, too low, and expected value; it verifies target sequence number does not increment for high/low and does increment for the expected sequence. Commit: db22a94.