fix(plugin-e2e): adapt alert rule advanced mode detection to Grafana 13.2

[tolzhabayev]

What this PR does / why we need it:

Grafana 13.2 removed the frontend usage of the alertingQueryAndExpressionsStepMode feature toggle (grafana/grafana#125086), which broke the nightly E2E run (29000729249) in two ways:

• AlertRuleEditPage.isAdvancedModeSupported() decided the editor mode by reading the toggle from boot data. On 13.2 the toggle is inert, so the page objects assumed the legacy editor while Grafana rendered the simplified step, and getQueryRow() timed out looking for a query row title that doesn't exist there. It now detects the mode from the DOM instead: wait for the query step to mount, then check for the advanced mode switch.
• alerting.basicMode.spec.ts forces the toggle off to test the legacy no-switch editor. That configuration no longer exists on >= 13.2, so the spec is skipped there. alerting.advancedMode.spec.ts covers the same scenarios on those versions.

Also makes the boot data override mutate the existing featureToggles and user objects in place instead of replacing them. The runtime config keeps a reference to the original object, so replacing it means plugins never see the override when the callback runs after config construction. This is what made queryEditor.tlsEnabled flaky on nightly.

Verified against grafana-enterprise nightly (13.2.0-28985753600, the build that failed in CI), 13.1.0 and 11.0.11: the alerting and feature-toggles suites are green on all three.

Which issue(s) this PR fixes:

n/a

[tolzhabayev]

Alternative to #2758

[leventebalogh]

LGTM 👍