Simplify operationRoutes payload in transaction route creation to use string array

[gandalf-at-lerian]

Context

The transaction route creation endpoint currently requires operationRoutes to be passed as an array of objects:

"operationRoutes": [
  { "operationRouteId": "{{or_pix_p2p}}" }
]

Previously, this field accepted a simpler format — a plain array of UUIDs:

"operationRoutes": [
  "0197f5a9-13bc-717a-a21c-f13eca9e289c",
  "0197f5a8-882d-752c-847b-b686ad442be0"
]

Goal

Revert operationRoutes to accept a string array of operation route IDs instead of an array of objects. This simplifies the API contract and aligns with the previous behavior.

Acceptance Criteria

• operationRoutes accepts string[] (array of UUIDs) in the transaction route creation payload
• Existing functionality (linking operation routes to transaction routes) is preserved
• Tests updated to reflect the new payload format