The fetchBuildDetails function was calling /v2/acts/{actorId}/builds/{buildId},
which is the actor-scoped endpoint and does not return inputSchema or
actorDefinition. This caused empty input schema objects for all actors.

Use the correct global endpoint /v2/actor-builds/{buildId} instead, which
contains the compiled build artifact data. Also add fallback to
actorDefinition.input (preferred) before inputSchema (deprecated legacy field).

Fixes: Empty/null inputSchema returns when fetching actor details via
fetch_actor_details.js

• Change endpoint: /v2/acts/{actorId}/builds/{buildId} → /v2/actor-builds/{buildId}
• Add actorDefinition.input as primary schema source
• Maintain backward compat with deprecated inputSchema field