Redundant backend validation in concurrent worker processes

[git-jxj]

Describe the bug

When a concurrent benchmark is initiated (e.g., with --rate-type=concurrent), the main process correctly creates and validates the backend.
However, when the worker processes are subsequently spawned for the test, each worker process calls the backend.validate() method again.

This results in multiple, redundant "Test connection" network requests being sent to the target endpoint: one for the main process, and one for each worker process.

Expected behavior

The backend validation should only be performed once by the main process before the workers are created. Worker processes, which receive a copy of the already-validated backend object, should not perform this validation again.

This would prevent unnecessary network requests, reduce benchmark startup latency, and make the logs cleaner.

Environment
Include all relevant environment information:

1. OS [e.g. Ubuntu 20.04]:
2. Python version [e.g. 3.12.2]:

To Reproduce

guidellm benchmark run
--target "http://your-api-endpoint/v1"
--rate-type=concurrent
--rate=2
--max-requests 2
--data='{"prompt_tokens":32,"output_tokens":32}'

Observe the console output. You will see the log message validating backend and the associated "Test connection" request logs appear multiple
times (once for the main process, and once for each of the 2 worker processes).

Errors
If applicable, add a full print-out of any errors or exceptions that are raised or include screenshots to help explain your problem.

Additional context
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[git-jxj]