fix(experiments): support result and creator list ordering

[justinsunyt]

Problem

Sorting the Experiments list by Result sent order=conclusion, which the API rejected, and sorting by Created by could raise a server error while building the database ordering expression.

Fixes PostHog#63621

Changes

• Allow conclusion / -conclusion as experiment list order fields and sort them in the same display order as the UI.
• Make created_by ordering use an explicit string output field and fall back to an empty value when the creator is missing.
• Add regression coverage for ascending and descending Result and Created by ordering.

How did you test this code?

As an agent, I tested this with:

• ruff format products/experiments/backend/experiment_service.py products/experiments/backend/presentation/views.py products/experiments/backend/test/test_presentation_api.py
• ruff check products/experiments/backend/experiment_service.py products/experiments/backend/presentation/views.py products/experiments/backend/test/test_presentation_api.py --fix
• hogli test products/experiments/backend/test/test_presentation_api.py --maxfail=1 -q -k "test_can_order_experiments_by_conclusion or test_can_order_experiments_by_created_by" with SANDBOX_PROVIDER unset for the local test environment
• Browser verification on local PostHog: clicked Result and Created by sorting on seeded Experiments rows and confirmed the URL/table updated without a load/server error.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

Docs update

No docs update needed; this fixes API sorting behavior for an existing UI.

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Captain Capy implemented the fix after reproducing the invalid conclusion order error and tracing the existing created_by ordering expression. The change keeps sorting server-side so paginated Experiment list requests remain consistent with the table headers.

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