security(app): reject file URLs in chat input

## Problem

The Chat API accepts `DataBlock` URL sources from request JSON. A `file://` source reaches media formatters, which read the referenced path on the service host before embedding the bytes in a request to the configured model provider.

The service example is network-facing and its current identity dependency trusts a request header, so a remote caller able to run an agent can request a service-host path instead of uploading media content.

## Scope

Reject `file://` URL sources at the `ChatRequest` boundary. This preserves trusted SDK and workspace flows that intentionally use local files, while keeping remote URLs and base64 uploads available to API clients.

## Regression coverage

Validate that a request containing a `file://` source is rejected during request parsing before the agent or any formatter can run.

Focus on the Chat API request parsing path where `ChatRequest` is built from JSON. Add validation at this boundary so `file://` URL sources in `DataBlock` are rejected before media formatters or agent execution. Add or update request-level tests for chat input parsing to assert that `file://` inputs fail while remote URLs/base64 uploads still pass. The issue is done when parsing returns a clear error for file URLs and downstream formatter/agent code is never reached.

Rédigé par le modèle d'indexation à partir du texte de l'issue.