fix: avoid mutating FunctionTool params_json_schema

[ioleksiuk]

Summary

Mirror the MCP fix in #3134 for the FunctionTool path. ensure_strict_json_schema is documented as mutating its input dict, but FunctionTool.__post_init__ currently passes self.params_json_schema directly. When users construct FunctionTool(...) with a schema dict they own (e.g. reused across instances), strict-mode side effects like additionalProperties: false and required: [...] get baked into the caller's dict.

The decorator-based function_tool(...) path is unaffected because function_schema() produces a fresh dict per call. The direct FunctionTool(...) constructor is documented and exported from agents.__init__, so callers who use it directly are exposed.

>>> shared = {"type": "object", "properties": {"x": {"type": "string"}}}
>>> ft = FunctionTool(name="t", description="d", params_json_schema=shared,
...                   on_invoke_tool=noop, strict_json_schema=True)
>>> shared
{'type': 'object', 'properties': {'x': {'type': 'string'}},
 'additionalProperties': False, 'required': ['x']}

Wrap the schema in copy.deepcopy() before passing to ensure_strict_json_schema, matching the MCP precedent at `src/agents/mcp/util.py:286`.

Test plan

• New regression test `test_function_tool_does_not_mutate_params_json_schema` (mirrors `tests/mcp/test_mcp_util.py::test_to_function_tool_does_not_mutate_mcp_input_schema`).
• `uv run pytest tests/test_function_tool.py tests/mcp/test_mcp_util.py` — 112 passed.
• `uv run ruff check src/agents/tool.py tests/test_function_tool.py` — All checks passed.

Issue number

N/A — found while auditing for the same mutation-aliasing pattern that #3134 fixed.

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation (no doc changes needed)
• I've run `make lint` and `make format`
• I've made sure tests pass

[seratch]

@codex review

[chatgpt-codex-connector]

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