[BUG] Subagent Model Selection not compatible with LLM Proxy with custom/short model names

Environment

• Platform (select one):
• [ ] Anthropic API
• [ ] AWS Bedrock
• [*] Google Vertex AI
• [*] Other: LiteLLM
• Claude CLI version: 1.0.65 (Claude Code)
• Operating System: Macos 15.5
• Terminal: Zsh

Bug Description

Using a custom model name in the model section of a custom agent leads to the model being ignored, and the default model being used. Using the short name for a model, ie Haiku, leads to CC utilizing the date stamped model name, which some users may not have setup in their model proxy.
claude-3.5-haiku vs claude-3-5-haiku-20241022

Steps to Reproduce

1. Have a LiteLLM proxy setup with only the short model names, enterprise so not user configurable.
2. Create agent using Haiku
3. Invoke agent

Expected Behavior

1. Create an agent with Haiku
2. Invoke agent
3. Profit

Actual Behavior

1. Create agent using Haiku
2. Invoke agent
3. Red Wall of text