Outdated "Explore" subagent fallback model list

- Copilot Chat Extension Version: Not applicable; this concerns the built-in Explore subagent fallback in the VS Code/Copilot integration.
- VS Code Version: Current development/Insiders context (exact version not provided).
- OS Version: Not provided.
- Feature (e.g. agent/edit/ask mode): Agent with "Explore" subagent
- Selected model (e.g. GPT 4.1, Claude 3.7 Sonnet): Default fallback currently prioritizes `Claude Haiku 4.5 (copilot)`, followed by `Gemini 3 Flash (Preview) (copilot)` and `Auto (copilot)`.
- Logs: None provided.

Steps to Reproduce:

1. Use the Explore subagent without configuring `chat.exploreAgent.defaultModel` or the Copilot-specific `chat.exploreAgent.model` override.
2. Inspect or invoke the generated Explore agent configuration.
3. Observe that the fallback list in `extensions/copilot/src/extension/agents/vscode-node/exploreAgentProvider.ts` prioritizes the static list beginning with `Claude Haiku 4.5 (copilot)`.

The fallback list should be periodically evaluated/refreshed with broadly available models that balance codebase-research intelligence capability, speed, and cost. (e.g. at time of writing `GPT 5.4 mini` seems to outperform for all these tradeoffs)

https://github.com/microsoft/vscode/blob/dce00340cb3dfc63ff10d6d96be1999ce5ff45cb/extensions/copilot/src/extension/agents/vscode-node/exploreAgentProvider.ts#L19-L23

As subagent orchestration becomes more common, a refreshed fallback list would continually improve the default experience for users who do not configure a subagent model explicitly in the settings.