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.go

@@ -247,6 +247,9 @@ func (s *Server) AddTool(t *Tool, h ToolHandler) {
 		panic(fmt.Errorf("AddTool %q: missing input schema", t.Name))
 	}
 	if s, ok := t.InputSchema.(*jsonschema.Schema); ok {
+		if s == nil {
+			panic(fmt.Errorf("AddTool %q: input schema is nil", t.Name))
+		}
 		if s.Type != "object" {
 			panic(fmt.Errorf(`AddTool %q: input schema must have type "object"`, t.Name))
 		}
@@ -261,6 +264,9 @@ func (s *Server) AddTool(t *Tool, h ToolHandler) {
 	}
 	if t.OutputSchema != nil {
 		if s, ok := t.OutputSchema.(*jsonschema.Schema); ok {
+			if s == nil {
+				panic(fmt.Errorf("AddTool %q: output schema is nil", t.Name))
+			}
 			if s.Type != "object" {
 				panic(fmt.Errorf(`AddTool %q: output schema must have type "object"`, t.Name))
 			}
@@ -437,6 +443,9 @@ func setSchema[T any](sfield *any, rfield **jsonschema.Resolved, cache *SchemaCa
 		if err != nil {
 			return zero, err
 		}
+		if internalSchema == nil {
+			return zero, fmt.Errorf("schema is nil for type %v", rt)
+		}
 		*sfield = internalSchema
 
 		resolved, err := internalSchema.Resolve(&jsonschema.ResolveOptions{ValidateDefaults: true})
@@ -466,6 +475,10 @@ func setSchema[T any](sfield *any, rfield **jsonschema.Resolved, cache *SchemaCa
 		}
 	}
 
+	if internalSchema == nil {
+		return zero, fmt.Errorf("schema is nil for type %v", rt)
+	}
+
 	resolved, err := internalSchema.Resolve(&jsonschema.ResolveOptions{ValidateDefaults: true})
 	if err != nil {
 		return zero, err