Migrate typespec-vscode language client from push to pull configuration model

[iscai-msft]

Context

packages/typespec-vscode/src/tsp-language-client.ts uses the deprecated synchronize.configurationSection option of vscode-languageclient (LSP "push" configuration model):

const options: LanguageClientOptions = {
  synchronize: {
    // oxlint-disable-next-line typescript/no-deprecated
    configurationSection: "typespec",
    fileEvents: watchers,
  },
  ...
};

This surfaced as a deprecation warning after the vscode-languageclient bump in #11643. Migrating to the "pull" configuration model was left out of that dependency-update PR (the oxlint-disable remains) and is tracked here.

What's actually deprecated

Only the LSP push model via synchronize.configurationSection. The server (packages/compiler/src/server/client-config-provider.ts) already uses the pull API for the initial load:

const configs = await connection.workspace.getConfiguration("typespec"); // already pull

The only piece depending on the deprecated push is the change-notification handler, which reads the pushed payload:

connection.onDidChangeConfiguration((params) => { config = params.settings?.typespec; });

Config is consumed read-only in two spots in compile-service.ts (config?.lsp?.emit and config?.entrypoint); those are unaffected.

Proposed migration

1. tsp-language-client.ts — remove configurationSection: "typespec" (and its comments / oxlint-disable); keep fileEvents: watchers.
2. server.ts onInitialize — capture params.capabilities.workspace?.didChangeConfiguration?.dynamicRegistration.
3. client-config-provider.ts — when that capability is present, register for change notifications in the pull model:

   connection.client.register(DidChangeConfigurationNotification.type, { section: "typespec" });

   Registering with the section means vscode-languageclient still sends params.settings.typespec, so the existing onDidChangeConfiguration handler keeps working. (Alternatively, register section-less and re-pull via getConfiguration("typespec") inside the handler.)

This is the first dynamic client.register in the server, so guard it behind the capability so non-VS-Code consumers (playground / standalone CLI) don't error.

Testing

There are no automated tests for the config-sync path, so this needs manual verification in the VS Code extension:

• Settings load correctly on server startup.
• Settings live-update on change (e.g. toggling typespec.lsp.emit).
• Non-VS-Code language-server consumers still initialize without errors.

References

• Deprecated usage: packages/typespec-vscode/src/tsp-language-client.ts (configurationSection)
• Server side: packages/compiler/src/server/client-config-provider.ts
• Introduced/deferred in upgrade deps #11643