feat(core): enforce tenant isolation guard outside production

Non-production environments (development/testing) now force the
effective tenant to the reserved test tenant "llm_tester" (or a
test_-prefixed override) regardless of DEFAULT_USER misconfiguration:

- Config.effective_default_user only honors DEFAULT_USER outside
  production when it is llm_tester or test_-prefixed; anything else
  is forced to llm_tester (tenant_forced flags the override)
- Config refuses startup (validation error) when a non-production
  environment is explicitly configured with the production tenant
  jpmschweitzer
- get_user() applies the same guard at request-context resolution,
  so an explicit request for the production tenant in dev/test is
  forced to llm_tester with a warning log
- initialize_application() emits one loud startup log line
  (tenant_guard_active / tenant_guard_production) stating the
  effective tenant

Unit tests cover the dev/test/prod x default/explicit-user matrix.

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

src/core/context.py

@@ -41,6 +41,33 @@ current_conversation: ContextVar[str | None] = ContextVar(
 )
 
 
+def _apply_tenant_guard(user: str) -> str:
+    """
+    Enforce tenant isolation at request-context resolution.
+
+    In non-production environments the production tenant must never be
+    the effective user - a request that explicitly asks for it is forced
+    to the reserved test tenant instead (with a loud log line).
+    """
+    # Import here to avoid circular dependency
+    from src.core.config import PRODUCTION_TENANT, TEST_TENANT, Environment, config
+
+    if (
+        config.ENVIRONMENT != Environment.PRODUCTION
+        and user == PRODUCTION_TENANT
+    ):
+        from src.core.logging_config import get_logger
+
+        get_logger(__name__).warning(
+            "tenant_guard_forced",
+            environment=config.ENVIRONMENT.value,
+            requested_tenant=user,
+            forced_tenant=TEST_TENANT,
+        )
+        return TEST_TENANT
+    return user
+
+
 def get_user() -> str:
     """
     Get current user from request context.
@@ -48,6 +75,8 @@ def get_user() -> str:
     Returns:
         User identifier for the current request.
         Falls back to environment-aware default if not set.
+        In non-production environments the production tenant is never
+        returned - the tenant guard forces the reserved test tenant.
 
     Example:
         user = get_user()  # "llm_tester" (dev) or "jpmschweitzer" (prod)
@@ -55,7 +84,7 @@ def get_user() -> str:
     user = current_user.get()
     if user == _USER_NOT_SET:
         return get_default_user()
-    return user
+    return _apply_tenant_guard(user)
 
 
 def get_conversation_id() -> str | None: