fix(librarian): apply tenant guard to explicit user args in client

LibraryDeskClient._resolve_user only enforced non-empty: an explicit
user argument to any tenant-scoped method bypassed tatlock's tenant
guard entirely and went straight to library-desk, and padded values
were sent un-stripped on the wire.

Route the explicit-arg path through the same apply_tenant_guard() used
by context resolution and strip whitespace before the empty check, so
a non-production environment can never send the production tenant (or
a sanitization-collision variant) to library-desk, regardless of how
the user was supplied. Defense in depth - no in-repo caller passes an
explicit user today.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/agents/librarian/client.py

@@ -17,7 +17,7 @@ import httpx
 from pydantic import BaseModel, Field
 
 from src.core.config import config
-from src.core.context import get_user
+from src.core.context import apply_tenant_guard, get_user
 from src.core.logging_config import get_logger
 
 logger = get_logger(__name__)
@@ -271,14 +271,19 @@ class LibraryDeskClient:
         request must carry an explicit tenant (a missing user will 422).
         An empty tenant is a programming or configuration error - fail
         loudly here, before any bytes hit the wire.
+
+        Explicit user arguments are stripped and routed through the same
+        tenant guard as context resolution (get_user() already applies
+        it), so a dev environment can never send the production tenant -
+        or a sanitization-collision variant of it - to library-desk.
         """
-        effective = user if user is not None else get_user()
-        if not effective or not effective.strip():
+        effective = (user if user is not None else get_user()).strip()
+        if not effective:
             raise ValueError(
                 "library-desk request requires a non-empty user (tenant); "
                 "got an empty value from the caller or request context"
             )
-        return effective
+        return apply_tenant_guard(effective)
 
     async def _request_with_retry(
         self,