fix(api): keep the events schema switch when modifiers are cleared

[pauldambra]

Problem

• A project admin who clears the query modifiers also clears the native events table switch that PostHog staff set for that project.
• The project then falls back to the global setting or allowlist, and stops reading the table staff chose.
• The validator already strips useNewEventsSchema from dict updates, but PATCH {"modifiers": null} replaces the whole column.

Changes

• Clearing modifiers now keeps useNewEventsSchema. All other modifiers still clear.
• The team and project serializers merge modifiers through one shared helper, merge_modifiers, instead of two copies of the merge.
• Dict updates behave as before.

How did you test this code?

Test rationale: test_clearing_modifiers_keeps_use_new_events_schema sends a null PATCH through the environments and projects URLs, which use different serializers. Both cases failed before the fix. The existing test_modifiers_use_new_events_schema_is_not_client_writable covers dict updates only.

Release status

• No feature flag controls this change

Docs update

None. No document under docs/ covers this server-managed key.

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Agent: Claude Code (claude-opus-5-5) in PostHog Desktop

• Review on #99248 flagged this bug in code that the refactor only moved. This fix lands on master separately, so the refactor stays a pure move.
• The open PR search found no other fix.
• Rejected alternative: reject null with a 400. That would break clients that use null to reset their modifiers.
• Skills: adopting-prs, paul-pair, writing-tests, improving-drf-endpoints, writing-pr-descriptions.

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Created with PostHog Desktop

🤖 Generated with Claude Code

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[github-actions]

🤖 CI report

⚠️ Trunk lane — backend Python lane

This PR is assigned to the backend Python lane. It runs backend Python tests and may merge in parallel with PRs in other lanes.

✅ Duplication (Python) — clean

New Python code duplication introduced by this branch. Fails at 70+ tokens in app code, or 150+ tokens when both copies live in test files. Advisory while the gate proves itself: extract a shared helper instead of copying.

✅ Duplication (TypeScript) — clean

New TypeScript code duplication introduced by this branch. Fails at 70+ tokens in app code, or 150+ tokens when both copies live in test files. Advisory while the gate proves itself: extract a shared helper instead of copying.

[trunk-io]

   

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