Diagnostics no-longer work with the Xcode-beta toolchain

[BrendanThompson]

Reproduction

1. Create a new iOS App project in Xcode-beta (If you're using swiftly then run swiftly use xcode)
2. Run xcode-build-server
3. Add something that is known to be wrong e.g. NotText("Hello, world?")

Expected

• Should show an error in diagnostics

Actual

Doesn't show any errors at all

Logs

Below are the logs relevant from zed.log

2026-07-07T15:11:31+10:00 INFO  [lsp] starting language server process. binary path: "/usr/bin/sourcekit-lsp", working directory: "/Users/brendanthompson/Developer/Test", args: []
2026-07-07T15:11:31+10:00 ERROR [project::lsp_store] Failed to register server capabilities: missing field `legend`
2026-07-07T15:11:31+10:00 ERROR [project::lsp_store] Failed to register server capabilities: data did not match any variant of untagged enum DiagnosticServerCapabilities

System

Zed: v1.10.0+preview.315.8d56867088a4d4269bed518cdb1f913a037179de (Zed Preview) 
OS: macOS 27.0 (Build 26A5378j)
Memory: 128 GiB
Architecture: aarch64
Xcode: Version 27.0 beta 3