Add comprehensive nitrogen model test suite

Implemented 12 test cases covering all nitrogen model functionality:

**Basic Tests (6):**
- Initialization: Verifies pools initialized correctly and output data written
- Application and Pool Splitting: Verifies N splits 15%/85% to root/available
- Rate Limiting: Verifies accumulation limited by max_N_accumulation_rate
- Stress Factor Output: Verifies stress factor exists and is in [0,1]
- Remobilization: Verifies remobilization runs without crashes under stress
- Fruit Removal: Verifies nitrogen consumed during fruit growth

**Integration Test (1):**
- Full Growth Cycle: Complete simulation with periodic N applications

**Edge Cases (4):**
- Zero Nitrogen: Verifies no crash with 0 N, stress factor low
- Excessive Nitrogen: Verifies stress factor approaches 1.0, clamps correctly
- No Leaves: Verifies no crash with plant having no leaves
- Division by Zero Prevention: Verifies bug fixes prevent crashes

**Model Control (1):**
- Enable/Disable: Verifies nitrogen model toggle functionality

All 12 tests pass with 196 assertions. Tests verify both functionality
and the critical bug fixes (fruit scale factor tracking and division by
zero prevention).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: bnbailey-psl