feat: Making static prompt values in Rag/File Search configurable in Vector Store Config (#4368)

# What does this PR do?

- Enables users to configure prompts used throughout the File Search /
Vector Retrieval
- Configuration is defined in the Vector Stores Config so they can be
modified at runtime
- Backwards compatible, which means the fields are optional and default
to the previously used values

This is the summary of the new options in the `run.yaml`
```yaml
vector_stores:
  file_search_params:
    header_template: 'knowledge_search tool found {num_chunks} chunks:\nBEGIN of knowledge_search tool results.\n'
    footer_template: 'END of knowledge_search tool results.\n'
  context_prompt_params:
    chunk_annotation_template: 'Result {index}\nContent: {chunk.content}\nMetadata: {metadata}\n'
    context_template: 'The above results were retrieved to help answer the user\'s query: "{query}". Use them as supporting information only in answering this query.{annotation_instruction}\n'
  annotation_prompt_params:
    enable_annotations: true
    annotation_instruction_template: 'Cite sources immediately at the end of sentences before punctuation, using `<|file-id|>` format like \'This is a fact <|file-Cn3MSNn72ENTiiq11Qda4A|>.\'. Do not add
extra punctuation. Use only the file IDs provided, do not invent new ones.'
    chunk_annotation_template: '[{index}] {metadata_text} cite as <|{file_id}|>\n{chunk_text}\n'
```

<!-- If resolving an issue, uncomment and update the line below -->
<!-- Closes #[issue-number] -->

## Test Plan
Added tests.

---------

Signed-off-by: Francisco Javier Arceo <farceo@redhat.com>

src/llama_stack/core/resolver.py

@@ -3,6 +3,7 @@
 #
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
+
 import importlib
 import importlib.metadata
 import inspect
@@ -406,13 +407,17 @@ async def instantiate_provider(
         args = [provider_spec.api, inner_impls, deps, dist_registry, policy]
     else:
         method = "get_provider_impl"
+        provider_config = provider.config.copy()
 
+        # Inject vector_stores_config for providers that need it (introspection-based)
         config_type = instantiate_class_type(provider_spec.config_class)
-        config = config_type(**provider.config)
+        if hasattr(config_type, "__fields__") and "vector_stores_config" in config_type.__fields__:
+            provider_config["vector_stores_config"] = run_config.vector_stores
+
+        config = config_type(**provider_config)
         args = [config, deps]
         if "policy" in inspect.signature(getattr(module, method)).parameters:
             args.append(policy)
-
     fn = getattr(module, method)
     impl = await fn(*args)
     impl.__provider_id__ = provider.provider_id