test: cover auth-config helpers and oauth2 cache fix

I added 13 new tests across 5 files, covering the auth-config functions that lacked tests. Categories follow Normal / Boundary / Error where applicable.

`cj/toggle-auth-source-debug` flips state once and back through two toggles. `cj/oauth2-auto--plstore-read-fixed` gets three tests: cache miss reads then caches, cache hit skips `plstore-open` entirely, and the `unwind-protect` runs `plstore-close` even when `plstore-get` signals.

`cj/reset-auth-cache` covers no-prefix (skip `shell-command`), with-prefix success, and with-prefix shell failure (Emacs caches still clear). `cj/kill-gpg-agent` covers shell exit 0 and non-zero. `cj/clear-oauth2-auto-cache` covers bound-with-entries, bound-but-empty, and unbound. The unbound test restores the binding via `unwind-protect` so other tests in the same Emacs session don't void-variable.

I stubbed every boundary via `cl-letf` (`plstore-open`, `plstore-get`, `plstore-close`, `oauth2-auto--compute-id`, `auth-source-forget-all-cached`, `epa-file-clear-cache`, `shell-command`, `message`, `call-process`) and didn't stub any internal helpers.

15 auth-config tests pass together: 2 existing plus 13 new.

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