fix(expires_after): make sure multipart/form-data is properly parsed (#3612)

https://github.com/llamastack/llama-stack/pull/3604 broke multipart form
data field parsing for the Files API since it changed its shape -- so as
to match the API exactly to the OpenAI spec even in the generated client
code.

The underlying reason is that multipart/form-data cannot transport
structured nested fields. Each field must be str-serialized. The client
(specifically the OpenAI client whose behavior we must match),
transports sub-fields as `expires_after[anchor]` and
`expires_after[seconds]`, etc. We must be able to handle these fields
somehow on the server without compromising the shape of the YAML spec.

This PR "fixes" this by adding a dependency to convert the data. The
main trade-off here is that we must add this `Depends()` annotation on
every provider implementation for Files. This is a headache, but a much
more reasonable one (in my opinion) given the alternatives.

## Test Plan

Tests as shown in
https://github.com/llamastack/llama-stack/pull/3604#issuecomment-3351090653
pass.

llama_stack/providers/utils/files/form_data.py

@@ -0,0 +1,69 @@
+# 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.
+
+import json
+
+from fastapi import Request
+from pydantic import BaseModel, ValidationError
+
+from llama_stack.apis.files import ExpiresAfter
+
+
+async def parse_pydantic_from_form[T: BaseModel](request: Request, field_name: str, model_class: type[T]) -> T | None:
+    """
+    Generic parser to extract a Pydantic model from multipart form data.
+    Handles both bracket notation (field[attr1], field[attr2]) and JSON string format.
+
+    Args:
+        request: The FastAPI request object
+        field_name: The name of the field in the form data (e.g., "expires_after")
+        model_class: The Pydantic model class to parse into
+
+    Returns:
+        An instance of model_class if parsing succeeds, None otherwise
+
+    Example:
+        expires_after = await parse_pydantic_from_form(
+            request, "expires_after", ExpiresAfter
+        )
+    """
+    form = await request.form()
+
+    # Check for bracket notation first (e.g., expires_after[anchor], expires_after[seconds])
+    bracket_data = {}
+    prefix = f"{field_name}["
+    for key in form.keys():
+        if key.startswith(prefix) and key.endswith("]"):
+            # Extract the attribute name from field_name[attr]
+            attr = key[len(prefix) : -1]
+            bracket_data[attr] = form[key]
+
+    if bracket_data:
+        try:
+            return model_class(**bracket_data)
+        except (ValidationError, TypeError):
+            pass
+
+    # Check for JSON string format
+    if field_name in form:
+        value = form[field_name]
+        if isinstance(value, str):
+            try:
+                data = json.loads(value)
+                return model_class(**data)
+            except (json.JSONDecodeError, TypeError, ValidationError):
+                pass
+
+    return None
+
+
+async def parse_expires_after(request: Request) -> ExpiresAfter | None:
+    """
+    Dependency to parse expires_after from multipart form data.
+    Handles both bracket notation (expires_after[anchor], expires_after[seconds])
+    and JSON string format.
+    """
+    return await parse_pydantic_from_form(request, "expires_after", ExpiresAfter)