feat: expose layout model selection as a per-request option

[b-g-d]

Problem

The docling library supports 6 layout analysis models via LayoutOptions.model_spec (docling_layout_heron, docling_layout_heron_101, docling_layout_egret_medium, docling_layout_egret_large, docling_layout_egret_xlarge, docling_layout_v2), but docling-serve has no ergonomic way to select between them per-request.

Users must either:

• Accept the default (heron)
• Construct a raw layout_custom_config dict, which is undiscoverable and verbose

By contrast, ocr_engine already has a simple enum field for per-request selection.

Proposal

Add a layout_model enum field to ConvertDocumentsRequestOptions (mirroring the ocr_engine pattern):

{
  "options": {
    "layout_model": "docling_layout_egret_large",
    ...
  }
}

When set, a model validator expands it into the existing layout_custom_config dict, so no changes to docling_jobkit are needed. When layout_custom_config is explicitly provided, it takes precedence.

Additionally, a DOCLING_SERVE_DEFAULT_LAYOUT_MODEL env var would allow setting a server-wide default.

Implementation

PR: #540