fix: Add deploy concurrency option and preserve default work pool behavior for multi-deploy

[hemantmm]

closes: #19096

Summary

This PR introduces a --deploy-concurrency option to prefect deploy to control the number of deployments created concurrently when deploying multiple deployments.

Additionally, it preserves the existing behavior of applying the configured default work pool to deployment configurations that do not explicitly specify one, while avoiding unnecessary warnings when the default pool originates from settings.

Checklist

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve fail-fast behavior when concurrency is one

With the default concurrency=1, this schedules every deployment coroutine up front; if one _run_single_deploy raises, asyncio.gather propagates the exception but does not cancel the other awaitables, and releasing the semaphore can let the next config enter _run_single_deploy before the CLI exits. In a multi-deploy where an earlier deployment fails, later deployments can still be created/updated, regressing the previous sequential fail-fast behavior and causing unexpected partial deploys after the reported failure.

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[codspeed-hq]

Merging this PR will not alter performance

✅ 8 untouched benchmarks
⏩ 1 skipped benchmark¹

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_{Comparing hemantmm:deploy-concurrency-default-work-pool (8877263) with main (8902f5b)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩