fix(tools): materialize input_examples iterable to retain examples across turns (#1990)

[iosayin]

Fixes #1990

Summary

When a generator or single-use iterator is passed as input_examples to beta_tool / beta_async_tool, BaseFunctionTool retained it directly. The first request/serialization consumed the iterator, causing subsequent requests in a conversation turn to serialize input_examples as [], silently losing the tool input examples.

Changes

• In BaseFunctionTool.__init__, materialized input_examples as a list if not None (list(input_examples) if input_examples is not None else None).
• Added a unit test in tests/lib/tools/test_functions.py verifying that generator-backed input_examples persist across repeated to_dict() serializations.