Add retry logic with exponential backoff for AgentRun RPC calls

[Copilot]

Plan: Add retry logic for AgentRun RPC calls

• Analyze the current AgentRun gRPC call flow and identify failure points
• Add retry configuration parameters to Config struct (max retries, initial backoff, max backoff)
• Implement retry logic with exponential backoff in grpc_client.go AgentRun method
• Fix import issues in grpc_test.go
• Add specific tests for retry logic (TestIsRetryableError)
• Address code review feedback (logging improvements, removed jitter comment)
• Improve error detection with gRPC status codes
• Fix backoff calculation to prevent integer overflow
• Run tests and security scan
• Final code review (no issues found)
• Merge main branch to incorporate latest changes

Summary

Successfully merged main branch which includes:

• Dependency updates: google.golang.org/grpc v1.77.0 → v1.78.0
• Dependency updates: github.com/golang-jwt/jwt/v4 v4.5.0 → v4.5.2
• Fix for missing types package causing test compilation failures

All tests pass after merge. The retry logic implementation remains intact and compatible with the latest codebase.

Original prompt

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This section details on the original issue you should resolve

<issue_title>AgentRun: rpc error: code = Unavailable desc = transport is closing</issue_title>
<issue_description>Describe the bug
AgentRun rpc calls sometimes fail on a selected node.

This is where this error comes from: https://github.com/distribworks/dkron/blob/master/dkron/grpc_agent.go#L92

because that one fails:
https://github.com/distribworks/dkron/blob/master/dkron/grpc_agent.go#L78

Which makes me think, that RPC call simply fails to an agent node there:
https://github.com/distribworks/dkron/blob/master/plugin/executor.go#L78

Also during that moment few other job executions intents on same node fail with same error.
I guess, some retries could be implemented there. Maybe fail happens, because agent can be busy with something at the moment, resulting in timeout firing on a caller side.

Error happens not often, but this results in missed executions.
Most probably that happens when there are lots of job execution intents at the same time to the same agent. (which is our case)

</issue_description>

<agent_instructions>Analyze the issue and propose a solution</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes AgentRun: rpc error: code = Unavailable desc = transport is closing #760

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[vcastellm]

@copilot merge main

[Copilot]

@copilot merge main

Done. Merged main branch (847ec74) which includes gRPC v1.78.0, JWT v4.5.2, and the types package fix. All tests pass.