Fix UDP loss summaries for legacy peers without an omit period

[JRWALKER]

• Version of iperf3:
  master

• Issues fixed:
  Addresses the no-omit UDP loss reporting case related to UDP loss figures incorrect and self-inconsistent against pre-3.13 peers (omitted_* substitution) #2055.
  Follow-up to Fix backward compatibility issue of client incorrectly print 0 UDP packets loss #1988; does not address all scenarios in UDP loss figures incorrect and self-inconsistent against pre-3.13 peers (omitted_* substitution) #2055.

• Brief description of code changes:
  When an older peer does not return omitted statistics, result parsing
  can assign the entire loss count to omitted_cnt_error. Per-stream text
  output handles the no-omit case separately, but SUM subtracts this
  value and incorrectly reports zero losses.

  Set omitted_cnt_error to zero when no omit period is configured.
  Preserve existing compatibility handling for tests with an omit period.

  Add regression tests for result exchange and reporting. Before the fix,
  four streams with 100 total lost packets incorrectly produce a zero-loss
  SUM.

• Validation:

  • All 5 make check tests passed on macOS.
  • Regression fixtures cover single/parallel streams, text/JSON,
    legacy/current result formats, and omit-period compatibility.
  • Local TCP/UDP smoke tests passed, including parallel, reverse,
    and omit-period cases.
  • Legacy-peer coverage uses simulated result messages rather than
    an actual older server binary.