[BUG] Replace bare print with warning on non-convergence, fix ValueError format

[tarun-227]

Summary

Two bugs found by code review:

1. print("did not converge") → warnings.warn() (pygam.py:821)

   • Libraries should not use print() for user-facing messages. Replaced with warnings.warn() so users can programmatically capture/filter non-convergence events via Python's warning system.

2. Malformed ValueError in _estimate_GCV_UBRE (pygam.py:1196-1199)

   • format(gamma) was passed as a separate argument to ValueError instead of being interpolated into the string. This produced a confusing tuple error message:

     # Before (broken):
     ValueError('gamma scaling should be greater than 1, but found gamma = {}', '0.5')
     
     # After (fixed):
     ValueError('gamma scaling should be greater than 1, but found gamma = 0.5')

Test plan

• Added test_non_convergence_emits_warning — verifies warning is emitted (not printed)
• Added test_GCV_UBRE_gamma_error_message — verifies error message contains the actual gamma value
• All 164 tests pass (162 existing + 2 new)

[Tushar8466]

The [MNT] fix ruff-format commit (3ce140a) is failing —
the ruff formatting check is not passing.

You may need to run ruff locally and fix the formatting before this can be merged:

ruff format pygam/
ruff check pygam/ --fix
git add .
git commit -m "fix: ruff formatting"
git push origin fix/convergence-warning-and-format-bug

Once that's resolved, this looks good to merge! ✅