[Backport M86] fix(chat): use DeepCodyModelRef instead of id

[sourcegraph-release-bot]

This commit renames DeepCodyAgentID to DeepCodyModelRef to align with the naming convention used for other model references. This change ensures consistency and clarity in the codebase.

Test plan

Agentic chat works:

<img width="613" alt="image" src="https://github.com/user-attachments/assets/75d5fce8-45bc-469f-96ee-63d73d575b1c" />

Before - agentic context is empty

<img width="609" alt="image" src="https://github.com/user-attachments/assets/302e1577-554b-462a-abea-d8814a368fd1" />

Backport 6aaceaf544c3f36d11738f67fd54bc19aac00ea4 from #7756