Add timeout to standard pilot fetch

[peter941221]

Summary

Fix StandardPilot.JobGetAvailable so a stalled fetch does not hang a producer indefinitely.

Problem

producer.dispatchWork intentionally strips cancellation from the work context before fetching jobs so an in-flight fetch is allowed to complete during shutdown:

• producer.go:744-766

That is reasonable, but StandardPilot.JobGetAvailable forwarded directly to exec.JobGetAvailable with no timeout at all:

• rivershared/riverpilot/standard_pilot.go:18-22

This meant a stalled driver call could block a standard-pilot producer forever. The pro pilot already applies per-attempt fetch timeouts, so the standard pilot was the outlier.

Change

Add a 10-second timeout inside StandardPilot.JobGetAvailable before calling the driver.

This keeps the existing shutdown semantics intact:

• fetches still ignore parent cancellation from dispatchWork
• but they are now bounded, so a wedged DB call eventually returns instead of freezing the producer forever

The timeout is local to the standard pilot so there is no driver SQL change and no producer state-machine change.

Testing

• added rivershared/riverpilot/standard_pilot_test.go
• covered MaxToLock <= 0 no-op behavior
• covered a hung JobGetAvailable call timing out with context.DeadlineExceeded
• covered parent cancellation still winning when the incoming context is already canceled

Verification

Locally verified with:

• GOPROXY=https://goproxy.cn,direct GOSUMDB=off go test ./rivershared/riverpilot -count=1

Closes #1026.