feat: add CMF description field and path-policy list include param

[SwarnimDoegar]

Summary

Mirrors image-server-dashboard PR #106 in the dashboard SDK spec (dashboard-sdk branch). Two independent changes.

1. Custom metadata field description

Optional description (string, max 500 chars), independent of the existing label/schema mutual-requirement.

• POST /api/v1/customMetadataFields — accept optional description.
• PATCH /api/v1/customMetadataFields/{id} — accept optional description (send an empty string to clear). Updated the summary and 400 error examples to match the backend.
• CustomMetadataField response schema — expose description (present only when set; not required).

2. Path-policy list include query param

• GET /api/clients/{clientNumber}/path-policy — added optional include query param (comma-separated; currently only customMetadataFields). When folderPath is not provided, include=customMetadataFields expands each policy's configured custom metadata fields. Unknown tokens are ignored.
• PathPolicyListItem.customMetadataFields — the array is now populated in two cases with different item shapes, modeled as a oneOf:
  • via folderPath → merged fields (PathPolicyCustomMetadataFieldMerged, augmented globals, selected only)
  • via include=customMetadataFields → the policy's raw stored overrides (PathPolicyCustomMetadataFieldInput)

Validation

• YAML parses (openapi: 3.1.0, info.version: 2.0.0).
• description present with maxLength: 500 in POST request, PATCH request, and the response component; absent from the component's required.
• label/schema required-if wording unchanged (no coupling to description).
• list_path_policies query params: folderPath, include; customMetadataFields items = oneOf[Merged, Input].
• All local $ref targets resolve.

No stainless-config/main.yaml change required (no new methods / renamed operationIds); the operationId sync script was not run since nothing it manages changed.