fix: pass is_error field to Anthropic API in tool results

[ameenalkhaldi]

Why are these changes needed?

The Anthropic model client drops the is_error field when converting FunctionExecutionResult to ToolResultBlockParam. The field is already populated correctly throughout the codebase — StaticWorkbench sets is_error=True on caught exceptions, AssistantAgent propagates it from tool results, and tests verify it's set. But tool_message_to_anthropic() never passes it through to the API.

Anthropic's ToolResultBlockParam natively supports is_error, and passing it lets Claude distinguish actual tool output from error messages, which improves its retry and error handling behavior.

Related issue number

Relates to #5267 (sub-task 2: explicit error field)

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally. (No doc changes needed — internal behavior fix, no new API surface.)
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

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