API should always return renderable request files (remove frontend fallback)

[vitormattos]

Description

The signature-position flow currently depends on a frontend fallback because the editable request payload may arrive without renderable files.
This increases frontend complexity and hides API contract inconsistencies.

Expected behavior

The API response used by the signature-position modal should always include at least one renderable file entry, with consistent file data (id, name, and resolvable file/url).

Acceptance criteria

1. Editable request payload always returns renderable files for Draft, Able to sign, and Partial signed states.
2. Frontend fallback logic can be removed without breaking PDF rendering.
3. Signature-position modal works without client-side reconstruction of file data.
4. Backend tests cover cases where source data has missing/empty files and still produce a valid normalized response.

Impact

Simpler frontend, clearer API contract, fewer edge-case failures in signature positioning.Continued with AutopilotSummary: I provided a concise English GitHub issue draft with title, short problem statement, expected behavior, acceptance criteria, and impact, focused on fixing the API contract so the frontend fallback can be removed.