fix(multiagent): guard MCPExecutionBinder map with RWMutex

Concurrent tool callbacks in eino_adk_run_loop and the MCP monitor
goroutine call Bind/ExecutionID on the same binder. The previous
implementation used a plain map[string]string, causing intermittent
'concurrent map read and map write' panics — the same race that
TestMCPExecutionBinder_ConcurrentBind was added to guard against.

Wrap the map with sync.RWMutex (write lock in Bind, read lock in
ExecutionID). The TrimSpace of toolCallID/executionID stays outside
the critical section to keep it minimal.

go vet ./internal/multiagent/...   ✓
go test -race ./internal/multiagent/... -run MCPExecutionBinder  ✓

internal/multiagent/mcp_execution_binder.go

@@ -1,9 +1,13 @@
 package multiagent
 
-import "strings"
+import (
+	"strings"
+	"sync"
+)
 
 // MCPExecutionBinder maps ADK toolCallID → MCP monitor execution ID for a single agent run.
 type MCPExecutionBinder struct {
+	mu         sync.RWMutex
 	byToolCall map[string]string
 }
 
@@ -20,12 +24,17 @@ func (b *MCPExecutionBinder) Bind(toolCallID, executionID string) {
 	if tid == "" || eid == "" {
 		return
 	}
+	b.mu.Lock()
 	b.byToolCall[tid] = eid
+	b.mu.Unlock()
 }
 
 func (b *MCPExecutionBinder) ExecutionID(toolCallID string) string {
 	if b == nil {
 		return ""
 	}
-	return b.byToolCall[strings.TrimSpace(toolCallID)]
+	tid := strings.TrimSpace(toolCallID)
+	b.mu.RLock()
+	defer b.mu.RUnlock()
+	return b.byToolCall[tid]
 }