fix: always send tenant headers in OpenViking _headers() when account/user are set

OpenViking 0.3.x requires X-OpenViking-Account and X-OpenViking-User headers for ROOT API key requests to tenant-scoped APIs. Previously the `!="default"` guard skipped these headers when account/user were the literal string "default", causing INVALID_ARGUMENT errors.

Remove the `!="default"` guard so headers are sent whenever account/user are truthy. Empty strings are still correctly skipped since `""` is falsy.

Update tests to reflect the new behavior:
- test_viking_client_headers_send_tenant_when_default: asserts "default" headers ARE present
- test_viking_client_headers_send_tenant_when_empty_falls_back_to_default: asserts "default" headers ARE present from constructor fallback

Based on #21775 by @happy5318

plugins/memory/openviking/__init__.py

@@ -100,18 +100,19 @@ class _VikingClient:
             raise ImportError("httpx is required for OpenViking: pip install httpx")
 
     def _headers(self) -> dict:
-        # Only send tenant headers when the user actually configured them.
-        # Legacy installs had account/user defaulted to the literal string
-        # "default" — treat that as unset so authenticated remote servers
-        # that derive tenancy from the Bearer key aren't overridden by a
-        # bogus tenant value.
+        # Always send tenant headers when account/user are configured.
+        # OpenViking 0.3.x requires X-OpenViking-Account and X-OpenViking-User
+        # for ROOT API key requests to tenant-scoped APIs — omitting them
+        # causes INVALID_ARGUMENT errors even when account="default".
+        # User-level keys can omit them (server derives tenancy from the key),
+        # but ROOT keys must always include them explicitly.
         h = {
             "Content-Type": "application/json",
             "X-OpenViking-Agent": self._agent,
         }
-        if self._account and self._account != "default":
+        if self._account:
             h["X-OpenViking-Account"] = self._account
-        if self._user and self._user != "default":
+        if self._user:
             h["X-OpenViking-User"] = self._user
         if self._api_key:
             h["X-API-Key"] = self._api_key