Track file extension in LSP request failure telemetry

[Copilot]

When an LSP request panics and the server recovers, the telemetry event includes no information about which file type was involved, making it harder to identify JS-specific or extension-specific patterns in failures.

Changes

• lsproto (generated + generator): Added optional fileExtension field to RequestFailureTelemetryProperties. Field is *string with omitzero so it's absent from the payload when not applicable.
• server.go: Updated recover to accept a fileExtension string parameter. All three generic handler registrars (registerLanguageServiceDocumentRequestHandler, registerLanguageServiceWithAutoImportsRequestHandler, registerMultiProjectReferenceRequestHandler) now extract the extension via tspath.TryGetExtensionFromPath(params.TextDocumentURI().FileName()) before the deferred call. handleCompletionItemResolve and handleCodeLensResolve do the same from their respective file references; handleWorkspaceSymbol passes "".
• telemetryReporting.ts: Added fileExtension?: string to LSErrorResponse to match.

Example telemetry properties emitted on a panic in a .js file:

{
  "errorCode": "-32603",
  "requestMethod": "textDocument.hover",
  "stack": "...",
  "fileExtension": ".js"
}

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