The A2aErrorDetail model used a field named 'details' for the optional
error payload, but JSON-RPC 2.0 §5.1 and the project specification both
require this field to be named 'data'.
Changes:
- Rename A2aErrorDetail.details -> data in src/cleveragents/a2a/models.py
- Add TDD BDD scenarios to features/a2a_jsonrpc_wire_format.feature that
verify A2aErrorDetail serializes with 'data' field (not 'details')
- Add corresponding step definitions in a2a_jsonrpc_wire_format_steps.py
No call sites used the 'details' keyword argument when constructing
A2aErrorDetail instances (all existing usages only set code and message),
so no other files required changes.
ISSUES CLOSED: #2745
freemo