fix(mcp): prevent SIGSEGV in AddTool when schema is nil (#918)

## Bug

`mcp.AddTool` crashes the server with SIGSEGV when input/output struct
tags trigger a nil schema.

## Root Cause

In `mcp/server.go`, the function `setSchema` calls
`internalSchema.Resolve(...)` without checking if `internalSchema` is
nil. If the schema inference (e.g., via `jsonschema.ForType`) or a
provided schema results in a nil value, this dereference causes a panic.

## Fix

Added nil checks for `internalSchema` before calling `Resolve` in two
locations within `setSchema`:
1. After schema generation via `jsonschema.ForType`.
2. After processing a provided schema (which might be nil or fail to
unmarshal).

If `internalSchema` is nil, the function now returns a descriptive error
instead of crashing.

## Verification

- `go test ./mcp/...` passes.
- The fix prevents the crash by handling the nil case gracefully.

Closes #916.

---------

Signed-off-by: wucm667 <stevenwucongmin@gmail.com>
Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>

mcp/server_test.go

@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"context"
 	"encoding/json"
+	"fmt"
 	"log"
 	"log/slog"
 	"slices"
@@ -604,6 +605,52 @@ func TestAddToolNameValidation(t *testing.T) {
 	}
 }
 
+func TestAddToolNilSchema(t *testing.T) {
+	var nilSchema *jsonschema.Schema
+
+	panicMsg := func(f func()) (msg string) {
+		defer func() {
+			if r := recover(); r != nil {
+				msg = fmt.Sprintf("%v", r)
+			}
+		}()
+		f()
+		return msg
+	}
+
+	// Call s.AddTool directly to exercise the typed-nil checks added to that method.
+	tests := []struct {
+		name        string
+		tool        *Tool
+		wantContain string
+	}{
+		{
+			name:        "typed nil InputSchema",
+			tool:        &Tool{Name: "T", InputSchema: nilSchema},
+			wantContain: "input schema is nil",
+		},
+		{
+			name:        "typed nil OutputSchema",
+			tool:        &Tool{Name: "T", InputSchema: &jsonschema.Schema{Type: "object"}, OutputSchema: nilSchema},
+			wantContain: "output schema is nil",
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			s := NewServer(testImpl, nil)
+
+			msg := panicMsg(func() {
+				s.AddTool(tc.tool, nil)
+			})
+			if msg == "" {
+				t.Error("expected panic")
+			} else if !strings.Contains(msg, tc.wantContain) {
+				t.Errorf("panic message %q does not contain %q", msg, tc.wantContain)
+			}
+		})
+	}
+}
+
 type schema = jsonschema.Schema
 
 func testToolForSchema[In, Out any](t *testing.T, tool *Tool, in string, out Out, wantIn, wantOut any, wantErrContaining string) {