Add ubuntu 26.04 (resolute) support

[yhaliaw]

Applicable spec:

Overview

Add support for building ubuntu 26.04 images.

Rationale

Needed for the new LTS ubuntu.

Module Changes

Checklist

• The charm style guide was applied
• The contributing guide was applied
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation for charmhub is updated.
• The PR is tagged with appropriate label (urgent, trivial, senior-review-required, documentation)
• The docs/changelog.md is updated with user-relevant changes in the format of keep a changelog v1.1.0
• The application version is incremented in app/pyproject.toml

[Copilot]

Pull request overview

Adds Ubuntu 26.04 (“resolute”) as a supported base image across the charm configuration, image-builder package, and CI/integration coverage so the project can build and validate 26.04 runner images.

Changes:

• Extend base-image/version mappings and enums to include 26.04 / resolute.
• Update image initialization/upload paths and test expectations to cover resolute.
• Adjust integration tests/commands and CI matrix to run (and selectively skip) checks on 26.04.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/state.py	Adds 26.04 -> resolute mapping for charm-side image selection.
charmcraft.yaml	Updates config description to mention resolute as supported.
app/src/github_runner_image_builder/config.py	Adds BaseImage.RESOLUTE and version/tag mapping for 26.04.
app/src/github_runner_image_builder/openstack_builder.py	Downloads/uploads resolute base image; adjusts external-script sudo logging behavior for 26.04; passes base image into script runner.
app/src/github_runner_image_builder/templates/cloud-init.sh.j2	Skips HWE kernel install on resolute; adds note about bbr on 26.04.
app/tests/unit/test_config.py	Adds unit coverage for BaseImage.RESOLUTE parsing/mapping.
app/tests/unit/test_openstack_builder.py	Updates expected cloud-init script content to match resolute-specific behavior.
app/tests/integration/helpers.py	Updates external script URL pin; adds resolute -> 26.04 mapping for integration metadata.
app/tests/integration/testdata/test_script.sh	Updates how secrets are preserved/passed in the external test script.
app/tests/integration/test_openstack_builder.py	Renames/adjusts secret keys used by the integration script/secrets checks.
app/tests/integration/commands.py	Skips HWE/bbr/journal URL checks on 26.04; updates secret assertions.
.github/workflows/integration_test_app.yaml	Adds resolute to the integration test matrix (amd64 only via excludes).

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[cbartz]

LGTM. Some comments.