nyc_taxis: widen random date range beyond January 1–15

`random_dates()` in `nyc_taxis/workload.py` generates pickup/dropoff ranges only within January 1–15, 2015. The full dataset covers the entire year, so queries consistently hit the same narrow index segment, skewing latency results.

## Fix
Extend the range to cover all of 2015, matching the actual corpus distribution.

**File:** `nyc_taxis/workload.py`

Open nyc_taxis/workload.py and inspect random_dates() first. Verify how its generated pickup/dropoff ranges are bounded, then confirm the completed behavior samples across the full 2015 dataset rather than only January 1–15.

Written by the indexing model from the issue text.