Fix custom image deployment via SimpleStreams (3.7)

[jlboostrun]

Problem

Custom images synced from a SimpleStreams boot source cannot be deployed. The deploy fails with:

Failed to retrieve curtin config: 'custom'

Launchpad bug: https://bugs.launchpad.net/maas/+bug/2164564

Root Cause

When custom images are synced from a SimpleStreams source, the importer stores them with a custom/ name prefix (e.g. custom/br-ubuntu-24.04) and rtype=SYNCED. However, all resource lookups for the custom osystem query with the bare series name and only match UPLOADED resources:

1. preseed.py:get_base_osystem_series — BootResource.objects.get(name=release, ...) never matches custom/{release}
2. bootresource.py:get_resource_for — name=series + rtype=(UPLOADED,) categorically excludes synced resources
3. rpc/boot.py:get_boot_config_for_machine — same name=final_series bare lookup in the xinstall path
4. rpc/boot.py:_get_files_map — same bare lookup for PXE kernel/initrd retrieval

This makes the custom osystem + SimpleStreams combination entirely undeployable. Only manually uploaded custom images (bare name, rtype=UPLOADED) work.

Fix

All four lookups now use name__in=[series, f"custom/{series}"] to match both naming conventions, and get_resource_for includes SYNCED in the rtype filter for custom osystem.

The change is backward-compatible: uploaded custom resources (bare name, UPLOADED) still match via the bare name in the name__in list.

Testing

• Existing tests pass (they use UPLOADED + bare names, still matched)
• New tests added:
  • test_get_resource_for_returns_synced_custom_resource — SYNCED resource with custom/ prefix found via bare series
  • test_get_boot_config_for_machine_synced_custom_image — full boot config lookup for SYNCED custom resource

Verification

Verified end-to-end on MAAS 3.7.2 (snap rev 41649): custom Ubuntu 24.04 image published to a SimpleStreams stream, synced to a MAAS controller, and deployed to a UEFI VM. Machine reached Deployed state with cloud-init complete.

The bind-mounted hot-fixes on the test controller used exactly these name lookups; this PR makes them persistent in the codebase.

[github-actions]

[maas-code-reviewer review]

LLM-generated review from https://github.com/canonical/maas-code-reviewer.
Intended to assist a human reviewer, not replace one — suggestions may be
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This change correctly identifies and fixes a bug where SYNCED custom images could not be deployed because the lookup logic only matched UPLOADED images with the bare series name.

However, I spotted a couple of issues:

1. A test class definition (TestGetAvailableCommissioningResources) was accidentally removed during the test additions, which messes up the test suite structure and causes unintended setUp overwrites.
2. In rpc/boot.py, replacing name=final_series with name__in=[...] introduces the possibility of BootResource.objects.get() throwing a MultipleObjectsReturned exception if a user happens to have both an uploaded and a synced custom image with the same series name. This exception currently goes unhandled.

Please see the inline comments for more details.