fix(tui-gateway): heal or fall back when a resumed session's provider is stale

A session row persists the provider identity a chat actually used. When that
provider is later renamed or removed (e.g. a custom_providers:/providers:
entry deleted, or a provider renamed oldone->newone), Desktop/TUI resume
restores the stale name into agent init and dies with:

  agent init failed: Unknown provider '<name>'

while the CLI resumes the same session fine with the configured default.

- runtime_provider: add is_routable_provider() (full resolution chain:
  built-in -> providers: -> custom_providers: -> models.dev)
- _stored_session_runtime_overrides: heal a non-routable provider via
  canonical_custom_identity (base_url -> model -> configured provider),
  drop to the configured default when unrecoverable, and clear the stale
  base_url after healing so a dead endpoint cannot override the registry URL
- _start_agent_build: gate deferred-resume overrides on provider routability;
  when the stored provider is gone, prefer the model the user picked for THIS
  session, else the configured default
- tests: is_routable_provider cases, heal/fallback round-trips, gate checks

Refs #75128

hermes_cli/runtime_provider.py

@@ -1104,6 +1104,45 @@ def canonical_custom_identity(
     return None
 
 
+def is_routable_provider(provider: Optional[str]) -> bool:
+    """Whether a provider name currently resolves to a routable route.
+
+    Empty/None is vacuously routable: agent build falls back to the
+    configured default instead of failing. A name that resolves through
+    the full chain (built-in -> user ``providers:`` -> ``custom_providers:``
+    -> models.dev) is routable; anything else would fail agent init with
+    "Unknown provider '<name>'".
+
+    Session resume uses this to detect a stale/renamed/removed provider
+    persisted in an older session snapshot, so recovery can fall back to
+    the configured default or the model the user picked instead of letting
+    the agent build die.
+    """
+    name = str(provider or "").strip()
+    if not name or name.lower() == "auto":
+        return True
+    if name.lower() == "custom":
+        # The bare string is the resolved billing class shared by every
+        # named custom entry — not a routable identity. restore paths must
+        # heal it (canonical_custom_identity) or fall back, never hand it
+        # straight to agent init.
+        return False
+    try:
+        from hermes_cli.providers import resolve_provider_full
+
+        config = load_config()
+        return (
+            resolve_provider_full(
+                name,
+                config.get("providers"),
+                get_compatible_custom_providers(config),
+            )
+            is not None
+        )
+    except Exception:
+        return False
+
+
 def _normalize_base_url_for_match(value) -> str:
     return str(value or "").strip().rstrip("/").lower()