fix: avoid mutating codex output schema input

[ioleksiuk]

Summary

Mirror the MCP fix in #3134 (and the `FunctionTool` fix in #3382) for the Codex `output_schema` path. `_resolve_output_schema` in `src/agents/extensions/experimental/codex/codex_tool.py:696-700` does `schema = dict(option)`, which is only a shallow copy — nested user dicts (e.g. entries in `option["properties"]`) are the same identity as the caller's. `ensure_strict_json_schema` (documented as mutating its input at `src/agents/strict_schema.py:20`) then mutates those nested dicts in place, leaking strict-mode side effects like `additionalProperties: false` and `required: [...]` back into the caller's stored schema.

```python

nested = {"type": "object", "properties": {"y": {"type": "string"}}}
option = {"type": "object", "properties": {"inner": nested}}
_resolve_output_schema(option)
nested
{'type': 'object', 'properties': {'y': {'type': 'string'}},
'additionalProperties': False, 'required': ['y']}
```

Switch to `copy.deepcopy(dict(option))` so the strict conversion only mutates the local copy, matching the precedent established by #3134 in MCP and #3382 in `FunctionTool`.

Test plan

• New regression test `test_codex_tool_resolve_output_schema_does_not_mutate_input` verifying both the top-level option dict and a nested object schema remain unmodified after `_resolve_output_schema` returns, and that the returned dict's nested entry is a fresh copy.
• `uv run pytest tests/extensions/experiemental/codex/test_codex_tool.py -k resolve_output_schema` — 3 passed.
• `uv run ruff check` on the touched files — All checks passed.

Issue number

N/A — found while auditing for the same mutation-aliasing pattern that #3134 and #3382 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]

Codex Review: Didn't find any major issues. Nice work!

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