fix(cdp): clear empty event ingestion filters

[lordspline]

Problem

Event ingestion filtering let users create drop rules, but deleting the last rule left the saved filter stuck because the UI treated the empty editor state as invalid instead of an intentional delete-all action.

Fixes PostHog#62507.

Changes

• Serialize filter trees with no condition leaves as a disabled empty config: mode=disabled, filter_tree=null, and no test cases.
• Keep partially completed non-empty conditions invalid so inline validation still reports All conditions must have a value.
• Normalize backend API/model persistence the same way so direct API saves of empty trees disable saved filters without changing tenant scoping or ingestion lookup behavior.
• Wrap the event filter upsert validation/save in a narrow transaction so invalid requests do not leave behind partial configs.
• Rehydrate filter_tree=null responses into the empty editor group so users can immediately add new filters again.

How did you test this code?

• Agent ran hogli test frontend/src/scenes/data-pipelines/event-filtering/eventFilterLogic.test.ts.
• Agent ran hogli test posthog/api/test/test_event_filter_config.py posthog/models/test/test_event_filter_config.py.
• Agent ran ruff check posthog/api/event_filter_config.py posthog/models/event_filter_config.py posthog/api/test/test_event_filter_config.py posthog/models/test/test_event_filter_config.py.
• Agent ran pnpm exec oxlint --quiet frontend/src/scenes/data-pipelines/event-filtering/eventFilterLogic.ts frontend/src/scenes/data-pipelines/event-filtering/eventFilterLogic.test.ts.
• Agent verified the running UI flow: create a dry-run event filter, save it, remove the final condition, save again, repeat save, and confirm the API persisted mode=disabled, filter_tree=null, and test_cases=[].
• Agent attempted pnpm --filter=@posthog/frontend typescript:check; it fails before reaching this change because the local checkout is missing generated Kea type files in unrelated paths such as src/exporter/ExporterLoginType and src/layout/FeaturePreviews/featurePreviewsLogicType.

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Automatic notifications

• Publish to changelog?
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Docs update

No docs update needed; this preserves the existing event ingestion filtering UI/API shape and fixes delete-all persistence semantics.

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Captain Capy implemented and verified this issue end to end. The key decision was to distinguish empty filter trees (intentional delete-all) from non-empty condition leaves with empty values (incomplete input), then enforce that semantic in both frontend serialization and backend persistence.

[capy-ai]

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