chore(core): refactor model resolution and cleanup fallback logic

[adamfweidman]

Summary

Standardizes model resolution by replacing getEffectiveModel with resolveModel across the core and CLI packages. This change also simplifies the fallback handler logic and renames internal client methods to better reflect their purpose (e.g., _getActiveModelForCurrentTurn).

Fixes: https://github.com/google-gemini/maintainers-gemini-cli/issues/1154

How to Validate

npm run preflight

Pre-Merge Checklist

• [ ] Updated relevant documentation and README (if needed)
• [x] Added/updated tests (if needed)
• [ ] Noted breaking changes (if any)
• [ ] Validated on required platforms/methods:
  • [ ] MacOS
    • [x] npm run
    • [ ] npx
    • [ ] Docker
    • [ ] Podman
    • [ ] Seatbelt
  • [ ] Windows
    • [ ] npm run
    • [ ] npx
    • [ ] Docker
  • [ ] Linux
    • [ ] npm run
    • [ ] npx
    • [ ] Docker