feat(surveys): add active matching surveys subscription

[AyobamiH]

Problem

Closes #1498.

getActiveMatchingSurveys is intentionally a one-shot query. For API surveys targeted by an event or action, the matching set can change after the query returns, but consumers currently have no push-based way to observe that change.

Changes

• Add posthog.onActiveMatchingSurveysChanged(callback), which returns an unsubscribe function.
• Emit the initial matching set after surveys load, then re-evaluate after event/action activation, cancellation, consumption, and session-scoped activation expiry.
• Keep getActiveMatchingSurveys one-shot to avoid changing existing callback semantics.
• Register cached event/action survey definitions during initialization so subscriptions also work before a background refresh.
• Suppress duplicate notifications when a repeated trigger does not change activation state.
• Isolate subscriber errors so one callback cannot interrupt capture processing.
• Add the public method to posthog-js, @posthog/types, and the production property-preservation list.

Test evidence

• corepack pnpm --filter posthog-js test:unit: 6,060 passed, 8 skipped.
• corepack pnpm --filter posthog-js test:functional: 10 passed.
• corepack pnpm --filter posthog-js typecheck
• corepack pnpm --filter posthog-js lint
• corepack pnpm --filter posthog-js format:check
• corepack pnpm --filter posthog-js build
• corepack pnpm --filter posthog-js bundle-size:array upstream/main: +1.54 KiB minified, +0.30 KiB gzip, +0.18 KiB Brotli.
• corepack pnpm --filter @posthog/types test:unit
• corepack pnpm --filter @posthog/types lint
• corepack pnpm --filter @posthog/types build
• corepack pnpm format
• corepack pnpm lint:oxlint
• corepack pnpm check:public-api

Release info Sub-libraries affected

Libraries affected

• All of them
• posthog-js (web)
• posthog-js-lite (web lite)
• posthog-node
• posthog-react-native
• @posthog/react-native-plugin
• @posthog/react
• @posthog/ai
• @posthog/convex
• @posthog/next
• @posthog/nextjs-config
• @posthog/nuxt
• @posthog/openfeature-node-provider
• @posthog/openfeature-web-provider
• @posthog/rollup-plugin
• @posthog/webpack-plugin
• @posthog/types
• @posthog/browser-common

Checklist

• Tests for new code
• Accounted for the impact of any changes across different platforms
• Accounted for backwards compatibility of any changes (no breaking changes!)
• Took care not to unnecessarily increase the bundle size

If releasing new changes

• Ran pnpm changeset to generate a changeset file

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

AyobamiH directed the issue selection, implementation, and maintainer-style review. Codex in ChatGPT Work Mode assisted with repository investigation, implementation, and verification.

The separate subscription API follows the maintainer's recommendation and preserves the existing getter's one-shot behavior. During review, the regression was strengthened to use an API survey and the real SurveyManager matching path without manually registering trigger definitions. A second regression preserves initial-notification ordering when an existing onSurveysLoaded callback captures the activating event.