fix(subagents): ensure model selection works for bare model IDs

- Handle bare model IDs by inheriting parent's authType
- Create dedicated ContentGenerator for any explicit model selection
- Add tests for model override scenarios

Previously, only cross-provider prefixed models (e.g., "openai:gpt-4o")
triggered ContentGenerator creation. Bare IDs like "qwen-coder" were
ignored, causing subagents to always use the parent's model.

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

docs/users/features/sub-agents.md

@@ -116,7 +116,7 @@ Use the optional `model` frontmatter field to control which model a subagent use
 - `inherit`: Use the same model as the main conversation
 - Omit the field: Same as `inherit`
 - `glm-5`: Use that model ID with the main conversation's auth type
-- `sonnet`, `opus`, `haiku`: Alias-style values are also accepted as model strings
+- `openai:gpt-4o`: Use a different provider (resolves credentials from env vars)
 
 #### Example Usage