fix(codex): respect user-owned model_catalog_json when generating catalog

[czfhhh]

Summary

Port the user-owned model-catalog fix from the desktop upstream (farion1231/cc-switch#6087). When model mapping generates the Codex catalog, cc-switch-cli currently replaces any existing model_catalog_json value with cc-switch-model-catalog.json. This PR makes the write side preserve user-managed catalog pointers.

Problem

set_codex_model_catalog_json_field in src-tauri/src/codex_config.rs has two arms:

• The None arm already checks ownership and only removes cc-switch-owned pointers.
• The Some arm unconditionally writes CC_SWITCH_CODEX_MODEL_CATALOG_FILENAME, clobbering user paths such as ~/.codex/model-catalog.local.json.

Starting the Codex local route with model mapping enabled therefore rewrites config.toml, breaking setups that depend on a user-managed catalog file. This builds on #265, which closed #260 by switching the same write to a relative filename.

Fix

Mirror the None arm's ownership rule in the Some arm: claim the pointer only when it is absent or already cc-switch-owned (matched by filename); otherwise leave the user's catalog path untouched. The generated cc-switch-model-catalog.json file is still written, so model-mapping based proxy routing keeps working.

Testing

• cargo fmt --check passes.
• cargo test --lib: 4334 passed, 0 failed, 2 ignored (CI-style sandbox environment).
• Added two unit tests covering the Some arm:
  • set_catalog_json_some_preserves_user_owned_catalog (absolute custom path)
  • set_catalog_json_some_preserves_user_owned_relative_filename (bare custom filename)
• Manually verified: with model mapping enabled and a user-owned model_catalog_json, enabling the Codex local route keeps the pointer in config.toml and multi-model routing continues to work.