Failure to activate on VS Code v1.100 #5573

@srawlins hit an issue with the extension activating today, which had an error very similar to #5570 but for the MCP server registration (not the lmTools registration).

The issue is that if the registerMcpServerDefinitionProvider API exists, we always call it, and we only look at the MCP flag within the provider (and return an empty set), which is to avoid telling VS Code in the manifest that we will register a provider and then not doing so.

However, on VS Code 1.100, this API exists but throws because it only proposed (and we're not allowed to use propsed APIs here).

So, we need a similar try/catch around this call.

This didn't occur for Firebase Studio because it's an older VS Code yet, and that didn't have the API at all.