bug: fix unreachable None guard in apikey_from_env

[NishchayMahor]

What

apikey_from_env has a if provider is None: return "" guard, but it sits after the line that dereferences provider, so it's unreachable — a None provider crashes instead:

def apikey_from_env(provider):
    env_key = f"{provider.upper().replace('-', '_')}_API_KEY"   # AttributeError if provider is None
    if provider is None:                                        # dead code — never reached for None
        return ""
    ...

apikey_from_env(None)   # AttributeError: 'NoneType' object has no attribute 'upper'

This is reachable through LLMOptions.parse_api_key, which calls apikey_from_env(values.get("provider", "")); constructing LLMOptions with provider=None (and no api_key/virtual_key) crashes with an opaque AttributeError instead of the intended empty-string behavior.

Fix

Move the None check above the dereference, so the guard actually runs:

def apikey_from_env(provider):
    if provider is None:
        return ""
    env_key = f"{provider.upper().replace('-', '_')}_API_KEY"
    ...

Testing

Added tests/test_utils.py (offline, no network/API key): asserts apikey_from_env(None) == "", plus positive cases (env lookup and dash normalization via monkeypatch.setenv). pytest tests/test_utils.py → 3 passed; the None case raises AttributeError on main without this change. black/ruff clean.