Fix missing tool export requirements for commented imports

[luojiyin1987]

Summary

Fix tool export requirement detection when imports include inline comments.

Closes #2211.

Changes

• parse imports via AST in get_imports() instead of regex
• keep excluding imports inside optional try/except blocks
• keep excluding flash-attn guarded imports
• include generated forward source when collecting requirements for SimpleTool
• add regression coverage for commented imports and function-tool save requirements

Verification

• python -m pytest tests/test_function_type_hints_utils.py -k 'test_get_imports'
• python -m pytest tests/test_utils.py -k 'function_tool_save'

[VANDRANKI]

Replacing the regex-based import detection with ast.parse() is the right fix. Regexes on Python source cannot correctly handle all comment forms (# noqa: F401, # type: ignore[import-not-found], etc.) without becoming arbitrarily complex. The AST approach is robust by construction.

visit() design is correct: the in_try_block and in_flash_attn_block flags propagate downward through child nodes, so any import nested inside a guarded block is correctly excluded. The ast.If detection for flash_attn guards uses re.fullmatch on the function name, which is more precise than the previous regex that operated on raw text.

requirements_sources.extend([tool_code, forward_source_code]) fixes a real bug: previously only tool_code was passed to get_imports(), but the forward function source could introduce additional imports not present in the class body. Now both are scanned.

# FIXME tests updated: the two tests that previously asserted {'smolagents'} with a FIXME comment now correctly assert {'IPython', 'smolagents'}. These are the best kind of regression tests - they came from a known bug.

Edge case: ast.parse(code) raises SyntaxError for invalid Python. The old regex approach would return [] (no imports found) for unparseable code. If get_imports() is ever called on code that could be syntactically invalid (e.g. templates with placeholder syntax), a try/except SyntaxError: return [] guard would maintain backward compatibility. Not a blocker since tool code is expected to be valid Python.

LGTM.