feat: add SourceNotConfiguredError and the governance backend reason

Pure addition ahead of the reparenting, so this commit leaves every
existing caller and test untouched.

SourceNotConfiguredError deliberately stays outside GatewayUnavailableError:
a source name that is not in the limiter's config dict is an assembly
defect, not a transient outage, and folding it into the retryable family
would let a typo retry forever without ever reaching a dead letter queue.

The retry_after_s default is 5.0 rather than 0 because a backlog released
at zero delay would stampede a backend that is already down.

tests/unit/test_errors.py

@@ -3,6 +3,8 @@
 import pytest
 
 from polygateway.errors import (
+    GOVERNANCE_BACKEND_RETRY_AFTER_S,
+    SCOPE_REASONS,
     AllSourcesExhausted,
     CircuitOpenError,
     GatewayUnavailableError,
@@ -11,6 +13,7 @@ from polygateway.errors import (
     RequestRejectedError,
     ResultInvalidError,
     SourceDeadError,
+    SourceNotConfiguredError,
     TransientError,
 )
 
@@ -89,3 +92,36 @@ class TestBackendFailure:
         exc = GovernanceBackendError("redis down")
         assert isinstance(exc, PolyGatewayError)
         assert not isinstance(exc, TransientError)
+
+
+class TestSourceNotConfigured:
+    """装配缺陷有意留在 scope 级家族之外(issue #7 §3.4,Q1 人类拍板)。"""
+
+    def test_is_domain_error_but_not_scope_level(self):
+        exc = SourceNotConfiguredError("未知源 'nope'(scope=llm)")
+        assert isinstance(exc, PolyGatewayError)
+        # 关键断言: 归入可重投家族会让配置写错的任务永远重投、永不进死信
+        assert not isinstance(exc, GatewayUnavailableError)
+
+    def test_exported_at_package_top_level(self):
+        import polygateway
+
+        assert polygateway.SourceNotConfiguredError is SourceNotConfiguredError
+        assert "SourceNotConfiguredError" in polygateway.__all__
+
+
+class TestGovernanceBackendReason:
+    """新 scope 级 reason 值域(issue #7 §3.1)。"""
+
+    def test_reason_admitted_to_scope_domain(self):
+        assert "governance_backend_down" in SCOPE_REASONS
+
+    def test_gateway_unavailable_accepts_the_new_reason(self):
+        exc = AllSourcesExhausted(
+            scope="LLM", reason="governance_backend_down", retry_after_s=0.0
+        )
+        assert exc.reason == "governance_backend_down"
+
+    def test_retry_after_default_is_non_zero(self):
+        """取 0 会让积压任务零延迟冲击已挂掉的后端(§3.2)。"""
+        assert GOVERNANCE_BACKEND_RETRY_AFTER_S > 0