fix: Improve error handling in MCP tool callbacks

- Add null check for isError() method in SyncMcpToolCallback
- Implement consistent error handling in AsyncMcpToolCallback to match SyncMcpToolCallback behavior
- Throw IllegalStateException with error content when tool calls fail

Resolves #2447

Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>
Co-authored-by: Christian Tzolov <christian.tzolov@broadcom.com>

Author: Fj-ivy

mcp/common/src/main/java/org/springframework/ai/mcp/AsyncMcpToolCallback.java

@@ -109,9 +109,12 @@ public String call(String functionInput) {
 		Map<String, Object> arguments = ModelOptionsUtils.jsonToMap(functionInput);
 		// Note that we use the original tool name here, not the adapted one from
 		// getToolDefinition
-		return this.asyncMcpClient.callTool(new CallToolRequest(this.tool.name(), arguments))
-			.map(response -> ModelOptionsUtils.toJsonString(response.content()))
-			.block();
+		return this.asyncMcpClient.callTool(new CallToolRequest(this.tool.name(), arguments)).map(response -> {
+			if (response.isError() != null && response.isError()) {
+				throw new IllegalStateException("Error calling tool: " + response.content());
+			}
+			return ModelOptionsUtils.toJsonString(response.content());
+		}).block();
 	}
 
 	@Override

mcp/common/src/main/java/org/springframework/ai/mcp/SyncMcpToolCallback.java

@@ -113,7 +113,7 @@ public String call(String functionInput) {
 		// Note that we use the original tool name here, not the adapted one from
 		// getToolDefinition
 		CallToolResult response = this.mcpClient.callTool(new CallToolRequest(this.tool.name(), arguments));
-		if (response.isError()) {
+		if (response.isError() != null && response.isError()) {
 			throw new IllegalStateException("Error calling tool: " + response.content());
 		}
 		return ModelOptionsUtils.toJsonString(response.content());