fix(llm): surface native retry attempts

[rekram1-node]

Summary

• emit native LLM retry events before internally retried HTTP requests
• convert native LLM errors into session API errors so existing retry policy can classify them
• mark transient transport failures retryable and publish retry state to the session UI

Tests

• bun typecheck (packages/llm)
• bun test test/executor.test.ts (packages/llm)
• bun typecheck (packages/opencode)
• bun test test/session/message-v2.test.ts test/session/retry.test.ts (packages/opencode)