Preserve more context in cross-project stack traces

@DanielRosenwasser 已经在做这个了。

开始于 2026年5月6日。

• #3726 来自 @copilot-swe-agent —— 已关闭，未合并

Today, if a `codeLens/resolve` request fails, we end up with a stack trace like

```
internal/ls.handleCrossProject[...]()
internal/ls/crossproject.go:210
internal/ls.(*LanguageService).ProvideReferences()
internal/ls/findallreferences.go:659
internal/ls.(*LanguageService).ResolveCodeLens()
internal/ls/codelens.go:65
internal/lsp.(*Server).handleCodeLensResolve()
internal/lsp/server.go:1590
internal/lsp.init.func1.registerRequestHandler[...].43()
internal/lsp/server.go:787
internal/lsp.(*Server).handleRequestOrNotification()
internal/lsp/server.go:648
internal/lsp.(*Server).dispatchLoop()
internal/lsp/server.go:527
```

`handleCrossProject` swallows up the rest of the context so we don't know the actual problem. We need a way to preserve the "real" stack.