Bugfix Issue 1919 planAddTC send_mail warning

Bugfix — Issue #1919: planAddTC save emits 3× E_WARNING "Undefined array key send_mail"

Symptom: On every POST /api/plans/index.php/planaddtc/save without send_mail (UI hides assignment panel when no build), three E_WARNING: Undefined array key "send_mail" rows were logged in the events table.

Root cause: In api/plans/index.php:1990, the boolean expression did three strict comparisons against the missing key after !empty() was false:

$sendMail = !empty($body['send_mail']) || $body['send_mail'] === true || $body['send_mail'] === 1 || $body['send_mail'] === "1";

Each === access of $body['send_mail'] when undefined emitted a warning.

Fix (minimal): api/plans/index.php:1990 → $sendMail = !empty($body['send_mail']);. Truth table preserved; no undefined key dereference.

Verification: Post-fix, saves without send_mail and with send_mail:false produced no new log_level=2 events. Regression test added to tmp/TLU_Test_Cases.md.

Files changed: api/plans/index.php:1990

Refs #1919