feat(garm): expose runner instance cleanup APIs

[yanksyoon]

Summary

• Add charm-side wrappers for listing runners in a scaleset.
• Add runner deletion wrapper with GARM force and GitHub authorization-bypass flags.
• Convert scaleset IDs to the generated client's required string type.

Test plan

• pytest -q tests/unit/test_garm_api.py

Part of the stacked implementation for ISD-6082. Follow-up cleanup orchestration PRs will target this branch.

Refs: ISD-6082

[Copilot]

Pull request overview

This PR extends the charms/garm charm-side Python wrapper around the generated GARM OpenAPI client to expose runner instance cleanup primitives (list instances for a scaleset + delete an instance with GARM cleanup flags), with accompanying unit tests. This supports the broader cleanup orchestration work for ISD-6082.

Changes:

• Add GarmAuthenticatedClient.list_scale_set_instances() wrapper that converts integer scaleset IDs to the generated client’s required string type.
• Add GarmAuthenticatedClient.delete_instance() wrapper exposing force_remove and bypass_gh_unauthorized flags.
• Add unit tests covering ID conversion and delete flag passthrough.

Reviewed changes

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

File	Description
charms/garm/src/garm_api.py	Adds authenticated wrapper methods around InstancesApi for listing scaleset instances and deleting an instance with cleanup flags.
charms/garm/tests/unit/test_garm_api.py	Adds unit tests validating scaleset ID string conversion and delete flag passthrough behavior.

Suppressed comments (1)

charms/garm/tests/unit/test_garm_api.py:273

• This new test’s docstring doesn’t follow the arrange/act/assert convention used throughout this module, which makes it harder to quickly scan what behavior is being verified.

    """Runner deletion exposes GARM's force and authorization-bypass flags."""

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