fix: #3270 Validate model retry backoff settings

[Aphroq]

Summary

Reject negative ModelRetryBackoffSettings values during construction or dict coercion. Negative initial_delay, max_delay, and multiplier values can otherwise produce negative retry delays, which the sleep helper treats as immediate retries.

Zero values remain valid explicit configuration.

Test plan

• ./.venv/bin/python -B -m pytest tests/models/test_model_retry.py
• bash .agents/skills/code-change-verification/scripts/run.sh

Issue number

Closes #3270

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass