test: Checkout gate-hook + capture_wallet + zero-settle paths reach 95% coverage

Adds tests covering the previously-uncovered Checkout flows:

- gate run_gate escape hatch (allow / deny / invalid-shape branches)
- gate per_request_policy returning None (skip-gate path)
- gate on_denied callback reshaping the canonical denial body
- gate happy path attaches ctx.capture_wallet closure; the closure itself
  is exercised end-to-end with AgentScoreCore.acapture_wallet mocked
- Checkout.accepted_rails dedupes tempo + tempo_session to a single slug
- Checkout.accepted_method_names emits each protocol method
- zero-settle MPP carve-out at zero (compose_mppx skipped, signer lifted)

Also tightens the _spec_rail_key / _spec_method_name return types from
RailKey | None / str | None to non-nullable (every code path returns a
value; the | None was dead and the key is None checks in the
accepted_rails / accepted_method_names accessors were unreachable).
Mirrors the same fix landed in node-commerce.

Tests: 1156 passed, 4 skipped. Total coverage: 93.02 to 95.00 percent.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: vvillait88

agentscore_commerce/checkout.py

@@ -104,34 +104,30 @@
 )
 
 
-def _spec_rail_key(spec: CheckoutRailSpec) -> RailKey | None:
+def _spec_rail_key(spec: CheckoutRailSpec) -> RailKey:
     """Map a ``*RailSpec`` instance to its canonical :data:`RailKey` slug.
 
     Tempo charge and Tempo session both speak MPP on Tempo, so they fold to
-    ``"tempo_mpp"``. Unknown types return ``None``.
+    ``"tempo_mpp"``.
     """
     if isinstance(spec, (TempoRailSpec, TempoSessionRailSpec)):
         return "tempo_mpp"
     if isinstance(spec, X402BaseRailSpec):
         return "x402_base"
     if isinstance(spec, SolanaMppRailSpec):
         return "solana_mpp"
-    if isinstance(spec, StripeRailSpec):
-        return "stripe"
-    return None
+    return "stripe"  # StripeRailSpec is the only remaining variant in CheckoutRailSpec.
 
 
-def _spec_method_name(spec: CheckoutRailSpec) -> str | None:
+def _spec_method_name(spec: CheckoutRailSpec) -> str:
     """Protocol-shaped method name for the ``methods: [...]`` discovery array."""
     if isinstance(spec, (TempoRailSpec, TempoSessionRailSpec)):
         return "tempo/charge"
     if isinstance(spec, X402BaseRailSpec):
         return "x402/exact (base)"
     if isinstance(spec, SolanaMppRailSpec):
         return "solana/charge"
-    if isinstance(spec, StripeRailSpec):
-        return "stripe/spt"
-    return None
+    return "stripe/spt"  # StripeRailSpec is the only remaining variant in CheckoutRailSpec.
 
 
 class CheckoutValidationError(Exception):
@@ -618,7 +614,7 @@ def accepted_rails(self) -> list[RailKey]:
         seen: set[str] = set()
         for spec in self.rails.values():
             key = _spec_rail_key(spec)
-            if key is None or key in seen:
+            if key in seen:
                 continue
             seen.add(key)
             out.append(key)
@@ -635,7 +631,7 @@ def accepted_method_names(self) -> list[str]:
         seen: set[str] = set()
         for spec in self.rails.values():
             name = _spec_method_name(spec)
-            if name is None or name in seen:
+            if name in seen:
                 continue
             seen.add(name)
             out.append(name)