fix: remove dummy data fallback in experiments_api.py render endpoint - #96

Fix dummy data fallback in experiments render endpoint

Problem: When no dataset is provided, the render figure endpoint was injecting dummy data [{"_": 1}] instead of letting generate_figure use the experiment's own metrics as fallback.

Solution: Pass data_override directly to generate_figure without the or [{"'_'": 1}] fallback. The function already handles None correctly by using experiment metrics.

Changes:

• backend/app/modules/platform/experiments_api.py: Removed dummy data fallback on line 436
• Added test to verify the pattern is not reintroduced

Testing: 2 tests pass, verifying the dummy pattern is absent and data_override is passed through correctly.