fix(providers): support Codex model discovery

[changjing9527]

Fixes #1992

Summary

• send the maintained client_version query parameter for Codex model discovery
• parse Codex's models[].slug / display_name response shape
• preserve the existing OpenAI-compatible discovery path unchanged
• add regression coverage for Codex mapping, deduplication, and query parameters

Validation

• uv run --frozen pytest tests/unit/test_provider_fetch_models.py -q (10 passed)
• uv run --frozen ruff check src/octop/infra/agents/providers/probe.py tests/unit/test_provider_fetch_models.py
• uv run --frozen ruff format --check src/octop/infra/agents/providers/probe.py tests/unit/test_provider_fetch_models.py
• git diff --check

[changjing9527]

Superseded by the clean, upstream-based implementation in #2013.