Preserve iterable ToolError content

[sylvesterkaczmarek]

Summary

Preserve structured ToolError content when callers provide a one-shot iterable such as a generator.

BetaFunctionToolResultType explicitly accepts Iterable[BetaContent]. ToolError.__init__() currently iterates non-string content to build the exception message and then stores the same iterable on self.content. For generators, that consumes the iterable before the tool runner later reads exc.content, so the emitted tool_result loses the structured error content.

Fix

Materialize non-string ToolError content once before rendering the exception message. The same reusable collection is then retained on self.content for the runner.

String content remains unchanged.

Regression coverage

Adds focused tests verifying:

• generator-backed structured error content survives through tool_error_content(), the same path used by the runners;
• the stored content remains reusable after exception message construction;
• string ToolError behavior is unchanged.

The production change is three lines in Anthropic's hand-maintained tool helper.