fix(gateway): don't restore a bare billing provider as the resumed session's provider

`_stored_session_runtime_overrides` restored the session provider from
`billing_provider` when `model_config` had no explicit provider. For a
`custom:<name>` endpoint that only ran normal turns (no `/model` switch), the
persisted `billing_provider` is the bare billing bucket `"custom"`, which
`agent_init` treats as non-routable, so `session.resume` failed with
"No LLM provider configured" even though new chats and CLI `--resume` work.

Only restore an explicit `model_config.provider`; skip a bare billing bucket
(`auto`/`openrouter`/`custom`) so resume falls back to the configured default,
matching the CLI path.

Fixes #44022

tests/test_tui_gateway_server.py

@@ -971,6 +971,35 @@ def test_session_resume_passes_stored_runtime_to_agent(monkeypatch):
     assert server._sessions[runtime_sid]["model_override"] == captured["model_override"]
 
 
+def test_stored_session_runtime_overrides_skips_bare_billing_provider():
+    """A bare billing bucket ("custom"/"auto"/"openrouter") must not be restored as the
+    provider identity on resume. A custom endpoint that never used `/model` persists only
+    `billing_provider="custom"`; restoring that broke `session.resume` with "No LLM provider
+    configured" (agent_init treats it as non-routable). A real provider, or an explicit
+    `model_config.provider`, is still restored.
+    """
+    # Bare "custom" bucket, no explicit model_config.provider: no provider override restored.
+    ov = server._stored_session_runtime_overrides({"model": "my-model", "billing_provider": "custom"})
+    assert "provider_override" not in ov
+    assert ov["model_override"]["provider"] is None
+
+    for bare in ("auto", "openrouter", "custom"):
+        ov = server._stored_session_runtime_overrides({"model": "m", "billing_provider": bare})
+        assert "provider_override" not in ov
+
+    # A real provider in billing_provider is still restored.
+    ov = server._stored_session_runtime_overrides({"model": "m", "billing_provider": "anthropic"})
+    assert ov["provider_override"] == "anthropic"
+    assert ov["model_override"]["provider"] == "anthropic"
+
+    # An explicit routable provider in model_config wins over the bare billing bucket.
+    ov = server._stored_session_runtime_overrides(
+        {"model": "m", "billing_provider": "custom", "model_config": {"provider": "custom:myendpoint"}}
+    )
+    assert ov["provider_override"] == "custom:myendpoint"
+    assert ov["model_override"]["provider"] == "custom:myendpoint"
+
+
 def test_persist_live_session_runtime_preserves_resume_metadata(monkeypatch):
     updates = {}