fix(codex): deduplicate Responses tools when converting to OpenAI Chat Completions

- Merge `tools` and `additional_tools` through a single canonical traversal and emit each Chat Completions tool name once (first declaration wins).
- Rework custom-tool and single-tool classification to use merged declaration identity, preventing collisions from duplicate definitions.
- Make reverse qualified-name resolution consistent with the same merge order so split calls map to the tool that actually survived conversion.

Closes: #4788