Feature request: model_aliases mapping for gateway model names to canonical Codex model metadata

What variant of Codex are you using?

CLI, IDE Extension

What feature would you like to see?

### Summary

Please add first-class support for model alias mapping in Codex config, so enterprise gateway model names can map to canonical Codex model IDs for metadata lookup.

### Problem

In enterprise environments, gateways often expose internal/public model names that reflect security tiers, environment boundaries, or compliance profiles (for example: gpt-5-3-codex-public). These names intentionally differ from canonical model IDs (for example: gpt-5.3-codex).

Today, this causes metadata lookup mismatches and warnings such as:

Model metadata for gpt-5-3-codex-public not found. Defaulting to fallback metadata; this can degrade performance and cause issues.

Workarounds exist (explicit metadata overrides in config.toml, or maintaining a custom model_catalog_json), but both add operational burden and drift risk for enterprise-managed installs.

### Requested feature

Add a config-level alias mapping mechanism, e.g.:

[model_aliases]
gpt-5-3-codex-public = "gpt-5.3-codex"
gpt-5-3-codex-il5 = "gpt-5.3-codex"
gpt-5-3-codex-il6 = "gpt-5.3-codex"

### Expected behavior

1. If model matches an entry in model_aliases, Codex resolves metadata using the canonical mapped model ID.
2. Alias resolution should work with custom providers (model_provider + [model_providers.<name>]).
3. Keep precedence explicit and deterministic, ideally:

• explicit per-model overrides in config (model_context_window, model_max_output_tokens, etc.)
• then alias-resolved metadata
• then fallback metadata

1. Alias support should work across profile scopes / managed config use cases.
2. Logs should clearly indicate alias resolution (for troubleshooting/auditability).

### Why this helps

• Supports enterprise naming conventions without changing gateway public model names.
• Reduces repetitive config workarounds across user/project/managed configs.
• Avoids maintaining custom model catalogs just for naming translation.
• Improves consistency for large deployments with multiple security-tiered aliases.

### Related context

• Existing warning behavior discussion: https://github.com/openai/codex/issues/21070
• Related PR (warning behavior): https://github.com/openai/codex/pull/21090

Additional information

_No response_