test: refactor init-extensions tests to avoid using ops.testing.Context.charm_spec

[james-garner-canonical]

The ops.testing framework currently exposes the private _CharmSpec class via the Context.charm_spec attribute (canonical/operator#2131). Charm Tech will deprecate this attribute soon (canonical/operator#2219). Charmcraft currently uses charm_spec in the init-extensions tests, so this PR preemptively modernises the test to use State.from_context, avoiding the need to access Context.charm_spec.

The init-extensions tests run only in the Ubuntu 22.04 spread tests. While that job is currently failing, the failing tests do not appear to be the init-extensions tests:

2025-12-12 04:44:41 Successful tasks: 50
error: unsuccessful run
2025-12-12 04:44:41 Aborted tasks: 0
2025-12-12 04:44:41 Failed tasks: 3
    - google:ubuntu-22.04-64:tests/spread/smoketests/different-dir
    - google:ubuntu-22.04-64:tests/spread/smoketests/multi-base:basic
    - google:ubuntu-22.04-64:tests/spread/smoketests/multi-base:one_platform
2025-12-12 04:44:41 Failed task restore: 2
    - google:ubuntu-22.04-64:tests/spread/smoketests/multi-base:basic
    - google:ubuntu-22.04-64:tests/spread/smoketests/multi-base:one_platform

AFAICT the init-extensions tests pass, here's some relevant output from this run.

2025-12-12 03:40:14 Restoring google:ubuntu-22.04-64:tests/spread/commands/init-extensions:springboot (dec120311-758761)...
2025-12-12 03:40:28 Preparing google:ubuntu-22.04-64:tests/spread/smoketests/reactive:stable (dec120311-758761)...
2025-12-12 03:40:29 Executing google:ubuntu-22.04-64:tests/spread/smoketests/reactive:stable (dec120311-758761) (7/53)...
2025-12-12 03:40:34 Restoring google:ubuntu-22.04-64:tests/spread/commands/init-extensions:flask (dec120311-758749)...
2025-12-12 03:40:50 Preparing google:ubuntu-22.04-64:tests/spread/smoketests/reactive:two (dec120311-758749)...
2025-12-12 03:40:51 Executing google:ubuntu-22.04-64:tests/spread/smoketests/reactive:two (dec120311-758749) (8/53)...
2025-12-12 03:41:14 Restoring google:ubuntu-22.04-64:tests/spread/commands/init-extensions:expressjs (dec120311-758638)...
2025-12-12 03:41:14 Restoring google:ubuntu-22.04-64:tests/spread/commands/init-extensions:fastapi (dec120311-758711)...
2025-12-12 03:41:30 Preparing google:ubuntu-22.04-64:tests/spread/dependencies/ (dec120311-758711)...
2025-12-12 03:41:30 Preparing google:ubuntu-22.04-64:tests/spread/dependencies/ (dec120311-758638)...
2025-12-12 03:41:34 Restoring google:ubuntu-22.04-64:tests/spread/commands/init-extensions:go (dec120311-758738)...
2025-12-12 03:41:52 Preparing google:ubuntu-22.04-64:tests/spread/smoketests/multi-base:all (dec120311-758738)...
2025-12-12 03:41:55 Executing google:ubuntu-22.04-64:tests/spread/smoketests/multi-base:all (dec120311-758738) (9/53)...
2025-12-12 03:42:40 Restoring google:ubuntu-22.04-64:tests/spread/commands/init-extensions:django (dec120311-758757)...

---

• I've followed the contribution guidelines.
• I've signed the CLA.
• I've successfully run make lint && make test.
• I've added or updated any relevant documentation. (N/A)
• I've updated the relevant release notes. (N/A)

[lengau]

Thanks! We need this sort of modernisation in a lot of places ☺️

[james-garner-canonical]

The 3.5.0 ops release yesterday includes the deprecation of Context.charm_spec. Is anything further needed in this PR before merging?

[lengau]

@james-garner-canonical nope — I just didn't see the approval. Merging nowe!