fix: accept null tool arguments and bound HTTP resource use

Review follow-ups on the SDK migration.

An "arguments": null is what clients send for parameterless tools like get_me,
and what mcp-go accepted by returning a nil map. The new adapter rejected it
with InvalidParams, which broke those calls outright.

The /mcp endpoint took unlimited request bodies and never expired idle
sessions, so a peer that goes away without DELETE kept its session for the
process lifetime. Both are reachable before any token check, so neither can
stay unbounded; the body cap sits above the SDK default to leave room for the
base64 content create_or_update_file accepts.

Required() smuggled a bool through the property schema map and deleted it
again, colliding with the JSON Schema keyword of the same name. It now sets a
field on Property, so an object property can carry its own required list.

The tool contract fixture cost a manual regeneration step and four
hand-maintained counts on every tool change, and a snapshot freezes defects
rather than reporting them. Property assertions cover the same surface and
reject a duplicate tool name, a readOnlyHint that disagrees with the register
call, and a default that contradicts its own type or enum.

Co-Authored-By: Claude (Opus 5) <noreply@anthropic.com>

pkg/tool/definition.go

@@ -10,7 +10,7 @@ type Property struct {
 }
 
 // PropertyOption configures one property in a tool's input schema.
-type PropertyOption func(map[string]any)
+type PropertyOption func(*Property)
 
 // NewDefinition builds a tool definition without enabling SDK-side validation.
 func NewDefinition(name, description string, annotations *mcp.ToolAnnotations, properties ...Property) *mcp.Tool {
@@ -40,71 +40,67 @@ func NewDefinition(name, description string, annotations *mcp.ToolAnnotations, p
 }
 
 func String(name string, options ...PropertyOption) Property {
-	return newProperty(name, "string", false, options...)
+	return newProperty(name, map[string]any{"type": "string"}, options...)
 }
 
 func Number(name string, options ...PropertyOption) Property {
-	return newProperty(name, "number", false, options...)
+	return newProperty(name, map[string]any{"type": "number"}, options...)
 }
 
 func Boolean(name string, options ...PropertyOption) Property {
-	return newProperty(name, "boolean", false, options...)
+	return newProperty(name, map[string]any{"type": "boolean"}, options...)
 }
 
 func Array(name string, options ...PropertyOption) Property {
-	return newProperty(name, "array", false, options...)
+	return newProperty(name, map[string]any{"type": "array"}, options...)
 }
 
 func Object(name string, options ...PropertyOption) Property {
-	return newProperty(name, "object", true, options...)
+	return newProperty(name, map[string]any{"type": "object", "properties": map[string]any{}}, options...)
 }
 
-func newProperty(name, propertyType string, object bool, options ...PropertyOption) Property {
-	schema := map[string]any{"type": propertyType}
-	if object {
-		schema["properties"] = map[string]any{}
-	}
+func newProperty(name string, schema map[string]any, options ...PropertyOption) Property {
+	property := Property{name: name, schema: schema}
 	for _, option := range options {
-		option(schema)
+		option(&property)
 	}
-
-	required, _ := schema["required"].(bool)
-	delete(schema, "required")
-	return Property{name: name, schema: schema, required: required}
+	return property
 }
 
+// Required marks the property as required on the parent schema. It is not a
+// property-level keyword, so it never touches the emitted property schema.
 func Required() PropertyOption {
-	return func(schema map[string]any) {
-		schema["required"] = true
+	return func(property *Property) {
+		property.required = true
 	}
 }
 
 func Description(description string) PropertyOption {
-	return func(schema map[string]any) {
-		schema["description"] = description
+	return func(property *Property) {
+		property.schema["description"] = description
 	}
 }
 
 func Enum(values ...string) PropertyOption {
-	return func(schema map[string]any) {
-		schema["enum"] = values
+	return func(property *Property) {
+		property.schema["enum"] = values
 	}
 }
 
 func Default(value any) PropertyOption {
-	return func(schema map[string]any) {
-		schema["default"] = value
+	return func(property *Property) {
+		property.schema["default"] = value
 	}
 }
 
 func Minimum(value float64) PropertyOption {
-	return func(schema map[string]any) {
-		schema["minimum"] = value
+	return func(property *Property) {
+		property.schema["minimum"] = value
 	}
 }
 
 func Items(schema any) PropertyOption {
-	return func(propertySchema map[string]any) {
-		propertySchema["items"] = schema
+	return func(property *Property) {
+		property.schema["items"] = schema
 	}
 }