Add application validation for transmission counts to prevent overflow

[coderabbitai]

Context

In DialogTransmissionAppender.Append, transmission counts are cast to short when updating FromPartyTransmissionsCount and FromServiceOwnerTransmissionsCount:

dialog.FromPartyTransmissionsCount += (short)fromParty;
dialog.FromServiceOwnerTransmissionsCount += (short)fromServiceOwner;

Currently, a checked cast will be added to throw an exception on overflow, but this should be complemented with application-level validation that prevents the overflow scenario from occurring in the first place.

Proposed Solution

Add validation logic that:

1. Checks if the new transmission counts would exceed short.MaxValue (32,767) before performing the cast
2. Returns a clear validation error to the caller if the limit would be exceeded
3. Prevents the need to catch overflow exceptions during normal operation

This validation should be added at the command/DTO validation level where appropriate, so users receive a clear error message rather than an exception.

Backlinks

• PR: fix(perf): Add CreateTransmissionCommand, refactor common transmission plumbing #3162
• Review comment: fix(perf): Add CreateTransmissionCommand, refactor common transmission plumbing #3162 (comment)
• Requested by: @elsand