feat(models): add @extensible decorator to get_api_key()

[Krashnicov]

Problem

get_api_key() in models.py resolves API keys exclusively from OS environment variables via os.getenv(). This means external secrets backends (OpenBao/Vault, AWS Secrets Manager, Azure Key Vault, etc.) cannot provide API keys to LiteLLM without injecting them into the process environment — a workaround that is fragile, doesn't support cache TTLs, and bypasses the existing SecretsManager architecture.

While helpers/secrets.py factory functions were recently made extensible (#1246), the actual API key resolution path for chat/embedding/utility models goes through models.get_api_key() → dotenv.get_dotenv_value() → os.getenv(), completely bypassing SecretsManager.

Solution

Add the @extensible decorator to get_api_key(), enabling plugins to intercept API key resolution at the models_get_api_key_start extension point.

This is a 2-line change:

1. Import extensible from helpers.extension
2. Add @extensible decorator to get_api_key()

How it works

With @extensible, a plugin extension can:

• Set data["result"] in a _start hook to provide an API key from any backend, short-circuiting the default dotenv lookup
• Fall through to the default os.getenv() behaviour when no plugin is active
• Modify data["args"] or data["kwargs"] to alter the service name lookup

Backward Compatibility

No behavioural change for existing users. The @extensible decorator only adds extension hooks around the existing function. Without any registered extensions, get_api_key() behaves identically to before.

Motivating Use Case

OpenBao secrets backend plugin that needs to provide LiteLLM API keys from a KV v2 secrets engine, with TTL-based caching and automatic fallback to .env files.

Related

• Follows the same pattern established in feat(secrets): add @extensible decorator to secrets factory functions #1246 for secrets factory functions
• Extension point naming: models_get_api_key_start / models_get_api_key_end

[Krashnicov]

Why helpers/settings.py is included

models.py is imported at module level by settings.py (line 9: import models). Adding from helpers.extension import extensible to models.py creates a circular import:

settings.py → import models → helpers.extension → helpers.subagents
  → helpers.plugins → extension (not yet initialized) → AttributeError

The fix moves import models from top-level in settings.py into the three functions that actually use it: convert_out(), _get_api_key_field(), and _load_sensitive_settings(). This is standard Python circular-import resolution — settings.py was the unusual importer (top-level), while models.py is widely imported across the codebase and shouldn't have import restrictions.

Python caches modules after first load, so the deferred import has zero performance impact on subsequent calls. No behavioural change.