Poll asynchronous jobs before downloading example outputs

[Ray0907]

Summary

• add shared polling helpers for SGLang and MiniMax Open Platform jobs
• wait for terminal success before reading result fields or downloading output
• stop immediately on failed, cancelled, or unexpected states
• add configurable polling timeout and transient HTTP retries
• apply the flow to all 18 asynchronous README scripts

Why

The video endpoints return an asynchronous job ID. The examples previously
queried each job only once and then attempted to read or download its output,
which fails whenever the job is still queued or processing.

The defaults poll every 10 seconds for up to two hours. Users can override
POLL_INTERVAL_SECONDS, POLL_TIMEOUT_SECONDS, and the HTTP_* retry values
for their deployment.

Validation

• bash -n passed for all 19 scripts in scripts/readme
• mock responses verified queued-to-completed, failed, unexpected, and timeout paths
• verified all 18 async examples source the polling helper
• git diff --check

The SGLang status handling follows its documented asynchronous video flow:
https://docs.sglang.io/cookbook/diffusion/MiniMax/MiniMax-H3#4-generate-video-and-audio

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