fix(testflight): add-testers 409 for an already-added tester should be an expected negative

Summary

asc testflight groups add-testers fails for 86.6% of installs (59 distinct, 5.x, 14 days); 24 of those are HTTP 409 because the tester is already in the group. Re-running an add is the normal agent pattern and should not fail.

Proposed fix

Key on Apple's 409 code for the already-a-member case (capture the real body; the --if-exists series established the pattern: match code, then read back membership), report it as an expected negative or a skip receipt, and keep other 409s failing. Consider aligning with --if-exists skip|update|fail for consistency.

Acceptance criteria

• httptest with the real 409 body → exit 0 with a receipt saying the tester was already present
• other 409 codes still fail
• read-back confirms membership before reporting success