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.

tests/unit/providers/utils/test_form_data.py

@@ -0,0 +1,179 @@
+# 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 unittest.mock import AsyncMock, MagicMock
+
+from pydantic import BaseModel
+
+from llama_stack.providers.utils.files.form_data import (
+    parse_expires_after,
+    parse_pydantic_from_form,
+)
+
+
+class _TestModel(BaseModel):
+    """Simple test model for generic parsing tests."""
+
+    name: str
+    value: int
+
+
+async def test_parse_pydantic_from_form_bracket_notation():
+    """Test parsing a Pydantic model using bracket notation."""
+    # Create mock request with form data
+    mock_request = MagicMock()
+    mock_form = {
+        "test_field[name]": "test_name",
+        "test_field[value]": "42",
+    }
+    mock_request.form = AsyncMock(return_value=mock_form)
+
+    result = await parse_pydantic_from_form(mock_request, "test_field", _TestModel)
+
+    assert result is not None
+    assert result.name == "test_name"
+    assert result.value == 42
+
+
+async def test_parse_pydantic_from_form_json_string():
+    """Test parsing a Pydantic model from JSON string."""
+    # Create mock request with form data
+    mock_request = MagicMock()
+    test_data = {"name": "test_name", "value": 42}
+    mock_form = {
+        "test_field": json.dumps(test_data),
+    }
+    mock_request.form = AsyncMock(return_value=mock_form)
+
+    result = await parse_pydantic_from_form(mock_request, "test_field", _TestModel)
+
+    assert result is not None
+    assert result.name == "test_name"
+    assert result.value == 42
+
+
+async def test_parse_pydantic_from_form_bracket_takes_precedence():
+    """Test that bracket notation takes precedence over JSON string."""
+    # Create mock request with both formats
+    mock_request = MagicMock()
+    mock_form = {
+        "test_field[name]": "bracket_name",
+        "test_field[value]": "100",
+        "test_field": json.dumps({"name": "json_name", "value": 50}),
+    }
+    mock_request.form = AsyncMock(return_value=mock_form)
+
+    result = await parse_pydantic_from_form(mock_request, "test_field", _TestModel)
+
+    assert result is not None
+    # Bracket notation should win
+    assert result.name == "bracket_name"
+    assert result.value == 100
+
+
+async def test_parse_pydantic_from_form_missing_field():
+    """Test that None is returned when field is missing."""
+    # Create mock request with empty form
+    mock_request = MagicMock()
+    mock_form = {}
+    mock_request.form = AsyncMock(return_value=mock_form)
+
+    result = await parse_pydantic_from_form(mock_request, "test_field", _TestModel)
+
+    assert result is None
+
+
+async def test_parse_pydantic_from_form_invalid_json():
+    """Test that None is returned for invalid JSON."""
+    # Create mock request with invalid JSON
+    mock_request = MagicMock()
+    mock_form = {
+        "test_field": "not valid json",
+    }
+    mock_request.form = AsyncMock(return_value=mock_form)
+
+    result = await parse_pydantic_from_form(mock_request, "test_field", _TestModel)
+
+    assert result is None
+
+
+async def test_parse_pydantic_from_form_invalid_data():
+    """Test that None is returned when data doesn't match model."""
+    # Create mock request with data that doesn't match the model
+    mock_request = MagicMock()
+    mock_form = {
+        "test_field[wrong_field]": "value",
+    }
+    mock_request.form = AsyncMock(return_value=mock_form)
+
+    result = await parse_pydantic_from_form(mock_request, "test_field", _TestModel)
+
+    assert result is None
+
+
+async def test_parse_expires_after_bracket_notation():
+    """Test parsing expires_after using bracket notation."""
+    # Create mock request with form data
+    mock_request = MagicMock()
+    mock_form = {
+        "expires_after[anchor]": "created_at",
+        "expires_after[seconds]": "3600",
+    }
+    mock_request.form = AsyncMock(return_value=mock_form)
+
+    result = await parse_expires_after(mock_request)
+
+    assert result is not None
+    assert result.anchor == "created_at"
+    assert result.seconds == 3600
+
+
+async def test_parse_expires_after_json_string():
+    """Test parsing expires_after from JSON string."""
+    # Create mock request with form data
+    mock_request = MagicMock()
+    expires_data = {"anchor": "created_at", "seconds": 7200}
+    mock_form = {
+        "expires_after": json.dumps(expires_data),
+    }
+    mock_request.form = AsyncMock(return_value=mock_form)
+
+    result = await parse_expires_after(mock_request)
+
+    assert result is not None
+    assert result.anchor == "created_at"
+    assert result.seconds == 7200
+
+
+async def test_parse_expires_after_missing():
+    """Test that None is returned when expires_after is missing."""
+    # Create mock request with empty form
+    mock_request = MagicMock()
+    mock_form = {}
+    mock_request.form = AsyncMock(return_value=mock_form)
+
+    result = await parse_expires_after(mock_request)
+
+    assert result is None
+
+
+async def test_parse_pydantic_from_form_type_conversion():
+    """Test that bracket notation properly handles type conversion."""
+    # Create mock request with string values that need conversion
+    mock_request = MagicMock()
+    mock_form = {
+        "test_field[name]": "test",
+        "test_field[value]": "999",  # String that should be converted to int
+    }
+    mock_request.form = AsyncMock(return_value=mock_form)
+
+    result = await parse_pydantic_from_form(mock_request, "test_field", _TestModel)
+
+    assert result is not None
+    assert result.name == "test"
+    assert result.value == 999
+    assert isinstance(result.value, int)