feat: add formbricks.on/off event subscriptions, forwarded to the SDK before setup [ENG-1814]

[pandeymangg]

What & why

The published wrapper forwards a hard-coded method list, so the new event API shipping in js-core (formbricks/formbricks#9133, ENG-1814) would be undefined for npm users. This adds formbricks.on(event, handler) / off(event, handler) with typed formbricks_* event names and payloads.

Two behaviours matter more than the plumbing:

1. Subscriptions are forwarded to the SDK BEFORE setup() runs — the opposite order from the method queue, which flushes after. A subscription queued behind setup would always register too late to hear formbricks_setup_successful; this ordering is what makes it observable through the wrapper at all.
2. The unsubscribe returned by on() works across the load boundary — taken before the SDK script loads, it removes the pending entry; taken after, it reaches the real registry. off() covers both states too.

Also feature-detects older self-hosted instances whose js-core predates events: a console warning instead of a crash, and the rest of the SDK keeps working.

The playground gains an "Events" panel that subscribes to all five events before setup() — the permanent way to see formbricks_setup_successful, which a browser console can never catch.

⚠️ Merge order

Do not merge/release before the js-core side ships (formbricks/formbricks#9133 → epic → release). Until then a published wrapper would only ever hit the older-instance warning path.

How this was tested

• Unit: 47/47 ✅ — six new tests: the flush-before-setup ordering (asserted mechanically via mock invocation order), formbricks_setup_successful observed through a setup that emits, unsubscribe before load / after forwarding, off() on pending entries, and the legacy-instance warning path.
• Ordering revert-check: moving the flush after instance.setup() turns the ordering test red.
• Live run against a local Formbricks (localhost:3000) through the playground: all five events captured on the page panel — formbricks_setup_successful, formbricks_action_tracked, formbricks_survey_shown, both formbricks_response_submitted emits with the persisted responseId, and formbricks_survey_closed exactly once after the ending card auto-closed.
• pnpm build, check-types ✅; biome clean on the touched files.

Linear: https://linear.app/formbricks/issue/ENG-1814 · release checklist: ENG-1850

[coderabbitai]

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
94.8% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud