19404 running concurrency limit

[F4RAN]

closes #19404

Summary

This PR fixes an issue where setting a deployment concurrency limit while flows are already in RUNNING state does not immediately apply the limit to those running flows. The root cause is that SecureFlowConcurrencySlots orchestration rule only applies to flows transitioning to PENDING state, so flows already RUNNING never acquire leases when a limit is set. This fix modifies the update_deployment API endpoint to retroactively acquire concurrency slots and create leases for existing RUNNING flows when a limit is added or decreased, ensuring that no further flows can start until the number of running flows falls below the limit. This is particularly useful when a sudden spike in flow runs needs to be capped immediately, as previously this could only be achieved by setting the limit to 0 and increasing it again once flows drained. The implementation includes useful unit tests verifying both adding a limit and decreasing a limit scenarios.

Checklist

• This pull request references any related issue by including "closes <link to issue>"
  • If no issue exists and your change is not a small fix, please create an issue first.
• If this pull request adds new functionality, it includes unit tests that cover the changes
• If this pull request removes docs files, it includes redirect settings in mint.json.
  I've not removed any docs
• If this pull request adds functions or classes, it includes helpful docstrings.
  I've not added any new function or class

[codspeed-hq]

CodSpeed Performance Report

Merging #19863 will not alter performance

_{Comparing F4RAN:19404-running-concurrency-limit (c569d19) with main (f14ad4d)}

Summary

✅ 2 untouched

[chatgpt-codex-connector]

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