feat(mcp): preserve CallToolResult.isError flag in MCPToolResult

[Zelys-DFKH]

Description

Adds an optional isError field to MCPToolResult, set to True only when the underlying MCP tool returned CallToolResult.isError=True. This lets callers tell apart application-level errors (the tool ran but reported a failure) from protocol/transport errors (the tool never ran because of a client exception, timeout, or session issue). Right now both surface identically as status="error", which is what #1670 is asking to fix.

Behavior:

• Tool-reported error (CallToolResult.isError=True): result has status="error" and isError=True
• Protocol/client exception: result has status="error" and no isError key. This path goes through _handle_tool_execution_error and is unchanged.
• Successful call: status="success", no isError key

The field is NotRequired, so existing code reading MCPToolResult keeps working without changes.

Related Issues

Closes #1670

Documentation PR

N/A

Type of Change

New feature

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• Extended the existing test_call_tool_sync_status and test_call_tool_async_status parametrized tests to assert result.get("isError") == (True if is_error else None)
• Extended test_mcp_tool_result_type to confirm the field is absent by default and settable when provided
• Ran hatch run prepare (ruff + mypy + full test suite): 2524 passed, 4 skipped, no new warnings

No new docs needed — this is a small additive field on an existing TypedDict, already documented in the docstring on MCPToolResult.

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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