Add retries for transient provider errors

[schalkneethling]

Summary

• add agent-level modelRetries config with exponential backoff defaults
• retry transient provider failures by removing the failed assistant turn and continuing from the same user/tool context
• avoid retrying quota, auth, invalid-request, content-filter, and aborted failures

Fixes #184

Testing

• pnpm --dir packages/runtime run test model-retries.test.ts
• pnpm --dir packages/runtime run build
• pnpm --dir packages/runtime run test
• pnpm --dir packages/runtime run check:types

[astrobot-houston]

Thanks for the contribution! We're closing this PR and moving the conversation to the discussion: #189

We've moved to a model where bugs and feature proposals are discussed in issues/discussions before code review, so the community can help prioritize and shape the work. Your branch is linked from the new thread so the implementation isn't lost — please join us there to continue the conversation.

— astrobot 🤖