Codex Support Legacy setup for versions < 0.134

[AarushiShah-db]

Summary

Adds backward‑compatible Codex config writing. For Codex CLI >= 0.134.0, continue writing per‑profile ~/.codex/ucode.config.toml. For older Codex CLI, write to shared ~/.codex/config.toml while preserving unrelated profiles. Refactors provider
config to reduce duplication and simplifies version handling.

Key Changes

• src/ucode/agents/codex.py
  • Add legacy paths: LEGACY_CODEX_CONFIG_PATH, LEGACY_CODEX_BACKUP_PATH.
  • Detect layout via _use_legacy_layout() (parses agent_version('codex') vs MINIMUM_CODEX_VERSION).
  • Extract _provider_block(...) to centralize Gateway provider config.
  • Add render_legacy_overlay(...) to build shared‑file overlay (profile, [profiles.ucode], [model_providers.ucode-databricks]).
  • Write logic preserves non‑ucode profiles in shared config.
• Tests
  • tests/test_agent_codex.py: new cases for legacy overlay contents, preservation of other profiles, and layout detection.
  • tests/test_agents_init.py: remove strict “force update Codex” blockers; compatibility replaces hard stop.

Behavior Before

• Required Codex >= 0.134.0; older versions prompted/blocked.

Behavior After

• Works with both layouts:
  • Modern (>= 0.134.0): per‑profile ~/.codex/ucode.config.toml.
  • Legacy (< 0.134.0): shared ~/.codex/config.toml, keeping existing non‑ucode sections intact.