Support consecutive batch job runs in agent

[lahirujayathilake]

This PR introduces the changes to run Agents for batch job submissions along with interactive jobs.

• Builds a locking/assignment flow so each agent only grabs one pending job unit at a time, marks it in-progress/completed, and reports the resolved command.
• Adds the gRPC request/response handling so agents can poll for the next job unit, receive commands, and push back execution status.
• Expands the agent work service logic and DB repo queries to track remaining job units within a batch

[DImuthuUpe]

@lahirujayathilake let's move this to linkspan