fix(providers): stop writing API keys into process os.environ

OpenAICompatProvider already passes api_key into AsyncOpenAI. Mutating
shared os.environ leaked credentials across providers (gateway overwrite
and setdefault first-writer-wins).

Fixes #4784

tests/providers/test_provider_env_isolation.py

@@ -0,0 +1,48 @@
+"""Provider credentials must not leak through process-global os.environ."""
+
+from __future__ import annotations
+
+import os
+
+from nanobot.providers.openai_compat_provider import OpenAICompatProvider
+from nanobot.providers.registry import find_by_name
+
+
+def test_provider_init_does_not_mutate_shared_env_keys(monkeypatch) -> None:
+    """Multi-provider setups must not overwrite or pin each other's keys."""
+    monkeypatch.delenv("OPENAI_API_KEY", raising=False)
+    monkeypatch.delenv("OPENROUTER_API_KEY", raising=False)
+
+    openai_spec = find_by_name("openai")
+    openrouter_spec = find_by_name("openrouter")
+    assert openai_spec is not None and openrouter_spec is not None
+
+    OpenAICompatProvider(
+        api_key="sk-openai-secret",
+        default_model="gpt-4o",
+        spec=openai_spec,
+    )
+    OpenAICompatProvider(
+        api_key="sk-or-secret",
+        default_model="openrouter/auto",
+        spec=openrouter_spec,
+        api_base="https://openrouter.ai/api/v1",
+    )
+
+    assert "OPENAI_API_KEY" not in os.environ
+    assert "OPENROUTER_API_KEY" not in os.environ
+
+
+def test_provider_init_preserves_preexisting_env_keys(monkeypatch) -> None:
+    monkeypatch.setenv("OPENAI_API_KEY", "preexisting-user-key")
+
+    openai_spec = find_by_name("openai")
+    assert openai_spec is not None
+    provider = OpenAICompatProvider(
+        api_key="sk-from-config",
+        default_model="gpt-4o",
+        spec=openai_spec,
+    )
+
+    assert os.environ["OPENAI_API_KEY"] == "preexisting-user-key"
+    assert provider._api_key_for_client == "sk-from-config"