fix(library_client): improve initialization error handling and prevent AttributeError (#2944)

# What does this PR do?

- Initialize route_impls to None in constructor to prevent
AttributeError
- Consolidate initialization checks to single point in request() method
- Improve error message to be more helpful ("Please call initialize()
first")
- Add comprehensive test suite to prevent regressions

The library client now has better error handling when users forget to
call initialize(), showing a clear ValueError instead of confusing
AttributeError. All initialization validation is now centralized in the
request() method, with internal methods (_call_non_streaming,
_call_streaming, _convert_body) relying on this single check for
cleaner, more maintainable code.

closes #2943 

## Test Plan

`./scripts/unit-tests.sh`

tests/unit/distribution/test_library_client_initialization.py

@@ -0,0 +1,90 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+"""
+Unit tests for LlamaStackAsLibraryClient initialization error handling.
+
+These tests ensure that users get proper error messages when they forget to call
+initialize() on the library client, preventing AttributeError regressions.
+"""
+
+import pytest
+
+from llama_stack.distribution.library_client import (
+    AsyncLlamaStackAsLibraryClient,
+    LlamaStackAsLibraryClient,
+)
+
+
+class TestLlamaStackAsLibraryClientInitialization:
+    """Test proper error handling for uninitialized library clients."""
+
+    @pytest.mark.parametrize(
+        "api_call",
+        [
+            lambda client: client.models.list(),
+            lambda client: client.chat.completions.create(model="test", messages=[{"role": "user", "content": "test"}]),
+            lambda client: next(
+                client.chat.completions.create(
+                    model="test", messages=[{"role": "user", "content": "test"}], stream=True
+                )
+            ),
+        ],
+        ids=["models.list", "chat.completions.create", "chat.completions.create_stream"],
+    )
+    def test_sync_client_proper_error_without_initialization(self, api_call):
+        """Test that sync client raises ValueError with helpful message when not initialized."""
+        client = LlamaStackAsLibraryClient("nvidia")
+
+        with pytest.raises(ValueError) as exc_info:
+            api_call(client)
+
+        error_msg = str(exc_info.value)
+        assert "Client not initialized" in error_msg
+        assert "Please call initialize() first" in error_msg
+
+    @pytest.mark.parametrize(
+        "api_call",
+        [
+            lambda client: client.models.list(),
+            lambda client: client.chat.completions.create(model="test", messages=[{"role": "user", "content": "test"}]),
+        ],
+        ids=["models.list", "chat.completions.create"],
+    )
+    async def test_async_client_proper_error_without_initialization(self, api_call):
+        """Test that async client raises ValueError with helpful message when not initialized."""
+        client = AsyncLlamaStackAsLibraryClient("nvidia")
+
+        with pytest.raises(ValueError) as exc_info:
+            await api_call(client)
+
+        error_msg = str(exc_info.value)
+        assert "Client not initialized" in error_msg
+        assert "Please call initialize() first" in error_msg
+
+    async def test_async_client_streaming_error_without_initialization(self):
+        """Test that async client streaming raises ValueError with helpful message when not initialized."""
+        client = AsyncLlamaStackAsLibraryClient("nvidia")
+
+        with pytest.raises(ValueError) as exc_info:
+            stream = await client.chat.completions.create(
+                model="test", messages=[{"role": "user", "content": "test"}], stream=True
+            )
+            await anext(stream)
+
+        error_msg = str(exc_info.value)
+        assert "Client not initialized" in error_msg
+        assert "Please call initialize() first" in error_msg
+
+    def test_route_impls_initialized_to_none(self):
+        """Test that route_impls is initialized to None to prevent AttributeError."""
+        # Test sync client
+        sync_client = LlamaStackAsLibraryClient("nvidia")
+        assert sync_client.async_client.route_impls is None
+
+        # Test async client directly
+        async_client = AsyncLlamaStackAsLibraryClient("nvidia")
+        assert async_client.route_impls is None