Pin the decorator's refusal messages and describe two wire-base tests

The two `@deferred_model` refusals asserted only the exception type;
they now snapshot the message so a wording change is a deliberate diff
rather than a silent one. The first two wire-base tests gain the
provenance docstrings the rest of the file carries.

Author: maxisbey

tests/types/test_deferred.py

@@ -9,6 +9,7 @@
 import typing
 
 import pytest
+from inline_snapshot import snapshot
 from mcp_types import Implementation, Tool
 from mcp_types._deferred import deferred_model
 from mcp_types._types import MCPModel
@@ -87,10 +88,12 @@ class OwnsHook(BaseModel):
         def __pydantic_init_subclass__(cls, **kwargs: object) -> None:
             raise NotImplementedError  # never runs: the decorator refuses the class first
 
-    with pytest.raises(TypeError):
+    with pytest.raises(TypeError) as eager_error:
         deferred_model(Eager)
-    with pytest.raises(TypeError):
+    assert str(eager_error.value) == snapshot("@deferred_model expects Eager to set model_config['defer_build'] = True")
+    with pytest.raises(TypeError) as hook_error:
         deferred_model(OwnsHook)
+    assert str(hook_error.value) == snapshot("@deferred_model would replace OwnsHook's own __pydantic_init_subclass__")
 
 
 def test_unresolvable_model_falls_back_to_the_generic_signature() -> None:

tests/types/test_wire_base.py

@@ -7,6 +7,9 @@
 
 
 def test_wire_model_defers_the_pydantic_build_to_first_use() -> None:
+    """SDK-defined: a `WireModel` subclass creates its class without building a validator;
+    the first `model_validate` builds it, once."""
+
     class Probe(WireModel):
         x: int
 
@@ -16,6 +19,9 @@ class Probe(WireModel):
 
 
 def test_wire_root_model_defers_the_parameterized_base_and_the_alias() -> None:
+    """SDK-defined: a `WireRootModel[T]` alias defers both itself and its generic
+    parameterization, and still validates through the root on first use."""
+
     class Payload(WireModel):
         x: int