refactor(lightning): migrate SendCoinsInput validator to Pydantic v2

Replace the deprecated Pydantic v1 @validator('amount', pre=True,
always=True) with a v2 @model_validator(mode='after'). The model
validator always runs and can see both amount and send_all, preserving
the cross-field rule (and the always=True semantics that reject the
empty/default case). Removes the last Pydantic v1 deprecation warning.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/lightning/models.py

@@ -6,7 +6,7 @@ from typing import List, Optional, Union
 from deepdiff import DeepDiff
 from fastapi.param_functions import Query
 from loguru import logger
-from pydantic import BaseModel, validator
+from pydantic import BaseModel, model_validator
 from pydantic.types import conint
 
 import app.lightning.docs as docs
@@ -1320,12 +1320,10 @@ class SendCoinsInput(BaseModel):
         ),
     )
 
-    @validator("amount", pre=True, always=True)
-    def check_amount_or_send_all(cls, amount, values):
-        if amount is None:
-            amount = 0
-
-        send_all = values.get("send_all") if "send_all" in values else False
+    @model_validator(mode="after")
+    def check_amount_or_send_all(self):
+        amount = self.amount if self.amount is not None else 0
+        send_all = self.send_all
 
         if amount < 0:
             raise ValueError("Amount must not be negative")
@@ -1339,10 +1337,6 @@ class SendCoinsInput(BaseModel):
                 )
             )
 
-        if amount > 0 and not send_all:
-            # amount is set and send_all is false
-            return amount
-
         if amount > 0 and send_all:
             # amount is set and send_all is true
             raise ValueError(
@@ -1352,12 +1346,8 @@ class SendCoinsInput(BaseModel):
                 )
             )
 
-        if amount == 0 and send_all:
-            # amount is not set and send_all is true
-            return amount
-
-        # normally this should never be reached
-        raise ValueError("Unknown input.")
+        # valid: (amount > 0 and not send_all) or (amount == 0 and send_all)
+        return self
 
 
 class SendCoinsResponse(BaseModel):

tests/test_send_coins_input.py

@@ -0,0 +1,39 @@
+"""
+Characterization tests for SendCoinsInput.amount / send_all validation.
+
+These lock the behaviour of the amount/send_all cross-field rule while the
+Pydantic v1 `@validator` is migrated to v2. They must pass before and after
+the migration.
+"""
+
+import pytest
+from pydantic import ValidationError
+
+from app.lightning.models import SendCoinsInput
+
+
+def test_neither_amount_nor_send_all_is_rejected():
+    with pytest.raises(ValidationError):
+        SendCoinsInput(address="bc1qexample")
+
+
+def test_zero_amount_without_send_all_is_rejected():
+    with pytest.raises(ValidationError):
+        SendCoinsInput(address="bc1qexample", amount=0)
+
+
+def test_amount_and_send_all_together_is_rejected():
+    with pytest.raises(ValidationError):
+        SendCoinsInput(address="bc1qexample", amount=1000, send_all=True)
+
+
+def test_positive_amount_is_accepted():
+    m = SendCoinsInput(address="bc1qexample", amount=1000)
+    assert m.amount == 1000
+    assert m.send_all is False
+
+
+def test_send_all_without_amount_is_accepted():
+    m = SendCoinsInput(address="bc1qexample", send_all=True)
+    assert m.send_all is True
+    assert m.amount == 0