fix(codersdk/toolsdk): fix tool schemata (#17365)

Fixes two issues with the MCP server:
- Ensures we have a non-null schema, as the following schema was making
claude-code unhappy:

 
```
        "inputSchema": { "type": "object", "properties": null },
```


- Skip adding the coder_report_task tool if an agent client is not
available. Otherwise the agent may try to report tasks and get confused.

cli/exp_mcp.go

@@ -402,7 +402,9 @@ func mcpServerHandler(inv *serpent.Invocation, client *codersdk.Client, instruct
 	// Create a new context for the tools with all relevant information.
 	clientCtx := toolsdk.WithClient(ctx, client)
 	// Get the workspace agent token from the environment.
+	var hasAgentClient bool
 	if agentToken, err := getAgentToken(fs); err == nil && agentToken != "" {
+		hasAgentClient = true
 		agentClient := agentsdk.New(client.URL)
 		agentClient.SetSessionToken(agentToken)
 		clientCtx = toolsdk.WithAgentClient(clientCtx, agentClient)
@@ -417,6 +419,11 @@ func mcpServerHandler(inv *serpent.Invocation, client *codersdk.Client, instruct
 
 	// Register tools based on the allowlist (if specified)
 	for _, tool := range toolsdk.All {
+		// Skip adding the coder_report_task tool if there is no agent client
+		if !hasAgentClient && tool.Tool.Name == "coder_report_task" {
+			cliui.Warnf(inv.Stderr, "Task reporting not available")
+			continue
+		}
 		if len(allowedTools) == 0 || slices.ContainsFunc(allowedTools, func(t string) bool {
 			return t == tool.Tool.Name
 		}) {
@@ -689,6 +696,11 @@ func getAgentToken(fs afero.Fs) (string, error) {
 // mcpFromSDK adapts a toolsdk.Tool to go-mcp's server.ServerTool.
 // It assumes that the tool responds with a valid JSON object.
 func mcpFromSDK(sdkTool toolsdk.Tool[any]) server.ServerTool {
+	// NOTE: some clients will silently refuse to use tools if there is an issue
+	// with the tool's schema or configuration.
+	if sdkTool.Schema.Properties == nil {
+		panic("developer error: schema properties cannot be nil")
+	}
 	return server.ServerTool{
 		Tool: mcp.Tool{
 			Name:        sdkTool.Tool.Name,