`SWBBuildServer` returns `-32001 "No build description"` for requests in the post-handshake window before async build description generation completes

[shawnshi75]

Description

SWBBuildServer.initialize() kicks off build description generation as fire-and-forget work, then immediately returns the InitializeBuildResponse. Any BSP request that requires buildDescriptionID and arrives after the handshake completes but before generation finishes fails with ResponseError(-32001, "No build description") instead of being queued or awaited.

Steps to reproduce

We hit this with sourcekit-xcode-bsp without pre-warminng the build description. The current workaround requests a build description before starting the actual build server.

1. Bring up the bridge so sourcekit-lsp completes the BSP handshake with SWBBuildServer (build/initialize -> build/initialized).
2. As soon as the handshake completes, sourcekit-lsp issues workspace/buildTargets (it does not call workspace/waitForBuildSystemUpdates first on this path).
3. Because initialize() only scheduled build description generation and buildDescriptionID is still nil, the request fails with ResponseError(-32001, "No build description").
4. No recovery was possible after the initial failure

The race is timing-dependent on how long generation takes, so it reproduces more reliably on larger workspaces (longer generation window).

Expected behavior

A request arriving in the natural post-handshake window should not fail.

Notes

The generation duration depends on workspace size, which widens the window for larger projects.