fix: correctly identify ondemand challenges to prune

[hi-liang]

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[Copilot]

Pull request overview

This PR updates pruning behavior to correctly target on-demand (“dynamic”) challenge instances by selecting builds based on instancecount == DYNAMIC_INSTANCES rather than relying on a schema name prefix, and updates/extends tests accordingly.

Changes:

• Update Manager.Prune() to delete old instances by filtering on builds.instancecount == DYNAMIC_INSTANCES (instead of schema LIKE "manual-%").
• Refactor prune tests to parameterize build InstanceCount and validate dynamic vs fixed-pool behavior.
• Add a test ensuring age-based pruning releases reserved ports via ON DELETE CASCADE.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
cmgr/prune_test.go	Updates fixtures/tests to validate pruning selection by instance count and adds port-release coverage.
cmgr/api.go	Changes pruning SQL filter to target on-demand builds by instancecount.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.