feat(T4): payload builder finalizat + snapshot test

- app/payload.py rafinat: odometruFinal/odometruInitial string (initial gol -> null),
  evita capcana falsy `or ""` (pastreaza "0"), normalizare vin/nrInm/coduri,
  tipPrestatie niciodata trimis, obs/b64Image omise cand lipsesc
- tests/test_payload.py: 10 teste, inclusiv snapshot vs exemplul oficial din contract

Verify: pytest 39 passed (29 + 10).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_payload.py

@@ -0,0 +1,102 @@
+"""Teste T4 — payload builder (app.payload.build_rar_payload).
+
+Snapshot fata de exemplul oficial de request din docs/api-rar-contract.md +
+invariantii din contract (status FINALIZATA, fara tipPrestatie, odometru string,
+idPrezentare null, omitere obs/b64Image, normalizare).
+"""
+
+from __future__ import annotations
+
+from app.models import PrezentareIn
+from app.payload import build_rar_payload
+
+
+def _internal(**over) -> dict:
+    data = {
+        "vin": "WVWZZZ1KZAW000123",
+        "nr_inmatriculare": "B999TST",
+        "data_prestatie": "2026-06-15",
+        "odometru_final": "123456",
+        "prestatii": [{"cod_prestatie": "OE-1"}],
+    }
+    data.update(over)
+    return PrezentareIn(**data).model_dump()
+
+
+def test_snapshot_exemplul_oficial_din_contract():
+    """Payload == exemplul oficial postPrezentare (request) din contract."""
+    internal = {
+        "vin": "XXXXXXXXXXXXXXXXX",
+        "nr_inmatriculare": "B999GEN",
+        "data_prestatie": "2024-12-01",  # exemplul are 2024-07-25, dar acela e < min; folosim o data valida, restul identic
+        "odometru_final": "9999999",
+        "odometru_initial": None,
+        "prestatii": [{"cod_prestatie": "OE-1"}, {"cod_prestatie": "OE-2"}],
+        "sistem_reparat": "null",
+        "obs": "TEST",
+        "b64_image": "UklGRg==",
+    }
+    expected = {
+        "vin": "XXXXXXXXXXXXXXXXX",
+        "nrInmatriculare": "B999GEN",
+        "dataPrestatie": "2024-12-01",
+        "odometruFinal": "9999999",
+        "odometruInitial": None,
+        "prestatii": [
+            {"codPrestatie": "OE-1", "idPrezentare": None},
+            {"codPrestatie": "OE-2", "idPrezentare": None},
+        ],
+        "sistemReparat": "null",
+        "status": "FINALIZATA",
+        "obs": "TEST",
+        "b64Image": "UklGRg==",
+    }
+    assert build_rar_payload(PrezentareIn(**internal).model_dump()) == expected
+
+
+def test_status_mereu_finalizata():
+    assert build_rar_payload(_internal())["status"] == "FINALIZATA"
+
+
+def test_nu_trimite_tipprestatie():
+    assert "tipPrestatie" not in build_rar_payload(_internal())
+
+
+def test_odometru_final_este_string():
+    p = build_rar_payload(_internal(odometru_final="55000"))
+    assert p["odometruFinal"] == "55000"
+    assert isinstance(p["odometruFinal"], str)
+
+
+def test_omite_obs_si_b64_cand_lipsesc():
+    p = build_rar_payload(_internal())
+    assert "obs" not in p
+    assert "b64Image" not in p
+
+
+def test_odometru_initial_gol_devine_null():
+    assert build_rar_payload(_internal(odometru_initial=""))["odometruInitial"] is None
+    assert build_rar_payload(_internal())["odometruInitial"] is None
+
+
+def test_odometru_initial_prezent_string():
+    p = build_rar_payload(_internal(prestatii=[{"cod_prestatie": "R-ODO"}], odometru_initial="100000"))
+    assert p["odometruInitial"] == "100000"
+
+
+def test_prestatii_au_idprezentare_null():
+    p = build_rar_payload(_internal(prestatii=[{"cod_prestatie": "OE-1"}, {"cod_prestatie": "OE-3"}]))
+    assert p["prestatii"] == [
+        {"codPrestatie": "OE-1", "idPrezentare": None},
+        {"codPrestatie": "OE-3", "idPrezentare": None},
+    ]
+
+
+def test_normalizare_vin_nrinm_in_payload():
+    p = build_rar_payload(_internal(vin="  wvwzzz1kzaw000123 ", nr_inmatriculare=" b999tst "))
+    assert p["vin"] == "WVWZZZ1KZAW000123"
+    assert p["nrInmatriculare"] == "B999TST"
+
+
+def test_sistem_reparat_default_null():
+    assert build_rar_payload(_internal())["sistemReparat"] == "null"