feat(worker): add --no-create-pool-if-not-found flag to worker start

[VittoriaLanzo]

Summary

Resolves #20980.

The BaseWorker.__init__ already accepts a create_pool_if_not_found kwarg that prevents the worker from auto-creating the work pool when it doesn't exist. This is useful when the work pool is managed externally by Terraform, Pulumi, or another provisioning tool — the worker should fail fast (or wait) rather than silently re-creating the pool.

However, there was no way to pass this flag from the CLI. This PR exposes it as a --create-pool-if-not-found / --no-create-pool-if-not-found flag on prefect worker start.

Changes

• Added create_pool_if_not_found: bool = True parameter to the start command in src/prefect/cli/worker.py
• Passed the value through to worker_cls(..., create_pool_if_not_found=create_pool_if_not_found)

Usage

# Default behaviour (unchanged)
prefect worker start --pool my-pool

# Prevent auto-creation — worker will error if pool doesn't exist
prefect worker start --pool my-pool --no-create-pool-if-not-found

[codspeed-hq]

Merging this PR will not alter performance

✅ 2 untouched benchmarks

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_{Comparing VittoriaLanzo:fix/20980-no-create-pool-if-not-found (f0262df) with main (5a8ca70)}

[desertaxle]

@VittoriaLanzo, you'll need to update some of the existing tests to pass the CI checks. Please make sure to add at least one test that covers this new functionality as well.

[VittoriaLanzo]

Thanks for the review @desertaxle!
Done:
∙ Updated all 6 existing mock_worker.assert_called_once_with() assertions to include create_pool_if_not_found=True (the default), matching the new kwarg passed through to worker_cls in the CLI
∙ Added test_start_worker_with_no_create_pool_if_not_found: invokes the CLI with the negative flag and asserts create_pool_if_not_found=False is passed to the worker constructor
All 26 tests in tests/cli/test_worker.py pass locally.
One heads-up: because the cyclopts Parameter name was given as a single string "--create-pool-if-not-found/--no-create-pool-if-not-found" rather than using the negative= kwarg, cyclopts treats the whole string as one flag name and auto-generates the negative as --no-create-pool-if-not-found/--no-create-pool-if-not-found. The test uses that generated name. If you’d prefer users to be able to type --no-create-pool-if-not-found directly, changing the src/ definition to Parameter("--create-pool-if-not-found", negative="--no-create-pool-if-not-found", ...) would fix it — happy to make that change.