feat(vector-io): implement global default embedding model configuration (Issue #2729)

- Add VectorStoreConfig with global default_embedding_model and default_embedding_dimension
- Support environment variables LLAMA_STACK_DEFAULT_EMBEDDING_MODEL and LLAMA_STACK_DEFAULT_EMBEDDING_DIMENSION
- Implement precedence: explicit model > global default > clear error (no fallback)
- Update VectorIORouter with _resolve_embedding_model() precedence logic
- Remove non-deterministic 'first model in run.yaml' fallback behavior
- Add vector_store_config to StackRunConfig and all distribution templates
- Include comprehensive unit tests for config loading and router precedence
- Update documentation with configuration examples and usage patterns
- Fix error messages to include 'Failed to' prefix per coding standards

Resolves deterministic vector store creation by eliminating unpredictable fallbacks
and providing clear configuration options at the stack level.

tests/unit/common/test_vector_store_config.py

@@ -0,0 +1,26 @@
+# 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.
+
+from llama_stack.apis.common.vector_store_config import VectorStoreConfig
+
+
+def test_defaults():
+    cfg = VectorStoreConfig()
+    assert cfg.default_embedding_model is None
+    assert cfg.default_embedding_dimension is None
+
+
+def test_env_loading(monkeypatch):
+    monkeypatch.setenv("LLAMA_STACK_DEFAULT_EMBEDDING_MODEL", "test-model")
+    monkeypatch.setenv("LLAMA_STACK_DEFAULT_EMBEDDING_DIMENSION", "123")
+
+    cfg = VectorStoreConfig()
+    assert cfg.default_embedding_model == "test-model"
+    assert cfg.default_embedding_dimension == 123
+
+    # Clean up
+    monkeypatch.delenv("LLAMA_STACK_DEFAULT_EMBEDDING_MODEL", raising=False)
+    monkeypatch.delenv("LLAMA_STACK_DEFAULT_EMBEDDING_DIMENSION", raising=False)