fix(toolbox-core): treat MCP CallToolResult.isError as an error (not a plain string)

[Deeven-Seru]

Summary

When the Toolbox server returns a successful MCP tools/call response whose result has isError: true, the Python SDK currently flattens the content into a normal string result. This means callers cannot reliably distinguish success vs failure, and ADK callbacks cannot handle these failures as structured error output.

PR #596 (fix(adk): wrap tool execution errors into error output) only wraps Python-side exceptions raised during execution/validation. Server-side tool failures represented via MCP CallToolResult.isError=true do not raise exceptions today, so they bypass the ADK wrapper.

Affected Area

• packages/toolbox-core MCP transport result processing
  • packages/toolbox-core/src/toolbox_core/mcp_transport/transport_base.py (_process_tool_result_content)
  • versioned MCP clients that call it, e.g. packages/toolbox-core/src/toolbox_core/mcp_transport/v20251125/mcp.py

Repro (high level)

1. Run a Toolbox server with a tool that returns an MCP tool result with isError: true (common when the server reports a tool execution failure).
2. Invoke the tool via toolbox-core (and/or via toolbox-adk).

Actual

• SDK returns a normal string like "error processing request: ...".
• No exception is raised, so wrappers like toolbox-adk cannot convert it into { "is_error": true, ... }.

Expected

One of the following (project preference):

• Raise a typed exception in toolbox-core when MCP tool result isError=true, OR
• Return a structured error object (or otherwise expose an error signal) instead of a plain string.

Why this matters

Downstream frameworks (ADK, LangChain, LlamaIndex) need a consistent way to detect tool failures and run after-tool callbacks / recovery logic. Right now only exception-based failures are catchable.

Related

• PR: fix(adk): wrap tool execution errors into error output #596