Address review: default missing 'required' and fix variant count

- server.py: JSON Schema 'required' is optional, so default it to an
  empty list before membership testing. External schemas with only
  optional properties no longer raise KeyError at import time.
- README.md: greet_dynamic publishes the same schema as greet_pydantic,
  so the client bullet now says four typed variants, not three.

Author: Aaron-Oh

examples/stories/schema_validators/README.md

@@ -33,7 +33,8 @@ uv run python -m stories.schema_validators.client --http --server server_lowleve
   `greet_pydantic` variant. A `create_model` result is opaque to static type
   checkers, so a `TYPE_CHECKING` branch aliases it to a declared model of the
   same shape — the runtime still uses the dynamic class.
-- `client.py` — the listed `inputSchema` for the three typed variants nests a
+- `client.py` — the listed `inputSchema` for the four typed variants (including
+  the runtime `create_model` one, whose schema matches `greet_pydantic`) nests a
   `$defs`/`$ref` object with a `name` property; `greet_dict` publishes only
   `{"type": "object", "additionalProperties": true}` — no field validation.
 - `server_lowlevel.py` — the same schemas written by hand. There is no

examples/stories/schema_validators/server.py

@@ -44,8 +44,11 @@ class PersonDC:
     # `name`/`title`; at runtime the dynamic class below is what @mcp.tool() sees.
     PersonDynamic = PersonModel
 else:
+    # `required` is optional in JSON Schema — a schema of all-optional properties
+    # omits it — so default to an empty list rather than indexing it directly.
+    _required = PERSON_JSON_SCHEMA.get("required", [])
     _dynamic_fields: dict[str, Any] = {
-        field_name: (str, ... if field_name in PERSON_JSON_SCHEMA["required"] else field_schema.get("default"))
+        field_name: (str, ... if field_name in _required else field_schema.get("default"))
         for field_name, field_schema in PERSON_JSON_SCHEMA["properties"].items()
     }
     PersonDynamic = create_model("PersonDynamic", **_dynamic_fields)