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/annotation/annotation_test.go

@@ -1,20 +1,50 @@
 package annotation
 
-import "testing"
+import (
+	"encoding/json"
+	"maps"
+	"testing"
 
+	"github.com/modelcontextprotocol/go-sdk/mcp"
+)
+
+// The hints are what clients use to decide whether a tool needs confirmation, so
+// assert the encoded form: an omitted readOnlyHint reads as false either way, but
+// only the explicit form survives a client that checks for the key.
 func TestAnnotations(t *testing.T) {
-	readOnly := ReadOnly("Read")
-	if readOnly.Title != "Read" || !readOnly.ReadOnlyHint || readOnly.DestructiveHint != nil {
-		t.Errorf("ReadOnly() = %#v", readOnly)
-	}
-
-	write := Write("Write")
-	if write.Title != "Write" || write.ReadOnlyHint || write.DestructiveHint != nil {
-		t.Errorf("Write() = %#v", write)
-	}
-
-	destructive := Destructive("Delete")
-	if destructive.Title != "Delete" || destructive.ReadOnlyHint || destructive.DestructiveHint == nil || !*destructive.DestructiveHint {
-		t.Errorf("Destructive() = %#v", destructive)
+	for _, test := range []struct {
+		name        string
+		annotations *mcp.ToolAnnotations
+		want        map[string]any
+	}{
+		{
+			name:        "ReadOnly",
+			annotations: ReadOnly("Read"),
+			want:        map[string]any{"title": "Read", "readOnlyHint": true, "idempotentHint": false},
+		},
+		{
+			name:        "Write",
+			annotations: Write("Write"),
+			want:        map[string]any{"title": "Write", "readOnlyHint": false, "idempotentHint": false},
+		},
+		{
+			name:        "Destructive",
+			annotations: Destructive("Delete"),
+			want:        map[string]any{"title": "Delete", "readOnlyHint": false, "idempotentHint": false, "destructiveHint": true},
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			encoded, err := json.Marshal(test.annotations)
+			if err != nil {
+				t.Fatalf("json.Marshal() error = %v", err)
+			}
+			var got map[string]any
+			if err := json.Unmarshal(encoded, &got); err != nil {
+				t.Fatalf("json.Unmarshal() error = %v", err)
+			}
+			if !maps.Equal(got, test.want) {
+				t.Errorf("annotations = %s, want %v", encoded, test.want)
+			}
+		})
 	}
 }