Failed MCP tool results are sent to the Interactions API with is_error=False #7824

🔴 Required Information

Describe the Bug:
With Gemini(use_interactions_api=True), a failed MCP tool call is sent to the Interactions API as a successful result. _convert_part_to_interaction_content in models/interactions_utils.py sets is_error on a function_result step only when the function response has an error key. A failed MCP CallToolResult reports the failure with isError: true (is_error in MCP SDK 2.x dumps) and has no error key, so the step goes out with is_error=False.

AnthropicLlm had the same gap and now reads isError/is_error (#7664, #7665).

Steps to Reproduce:

1. Run an MCP server with a tool that fails (a raised exception becomes an isError: true result).
2. Give an LlmAgent with Gemini(..., use_interactions_api=True) an McpToolset for that server.
3. Have the model call the tool, then inspect the function_result step in the next interactions.create input.

Expected Behavior:
The function_result step has is_error=True.

Observed Behavior:

function_result sent to Gemini: name=read_file is_error=False result='Error executing tool read_file'

Environment Details:

• ADK Library Version (pip show google-adk): 2.13.0 (also on main at 75fb788)
• Desktop OS: macOS
• Python Version (python -V): 3.11.16

Model Information:

• Are you using LiteLLM: No
• Which model is being used: gemini-2.5-flash with use_interactions_api=True

🟡 Optional Information

Additional Context:

func_resp: object = part.function_response.response
is_error = False
if isinstance(func_resp, dict) and 'error' in func_resp:
  is_error = True

How often has this issue occurred?:

• Always (100%)