Allow other VS Code extensions to install LSP middleware on language feature responses

[Andrew Branch (andrewbranch)]

Closes #63800

The second commit moves types for our VS Code extension API into the typescript package, generating a big chunk of the language server protocol, since middleware registration exposes it.

Example usage:

import * as vscode from "vscode";
import type { ExtensionAPI } from "typescript/unstable/vscode";

export async function activate(context: vscode.ExtensionContext) {
    const extension = vscode.extensions.getExtension<ExtensionAPI>(
        "TypeScriptTeam.native-preview",
    );
    if (!extension) return;

    const extensionAPI = await extension.activate();
    context.subscriptions.push(
        extensionAPI.registerLspMiddleware("textDocument/hover", (hover, context) => {
            if (!hover || !context.params.textDocument.uri.endsWith(".vue")) {
                return hover;
            }
            return {
                ...hover,
                contents: { kind: "markdown", value: "Custom hover for Vue files" },
            };
        }),
    );
}

[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation is internally consistent, fault-isolated, and covered by focused runtime and generation tests.

Review effort: Balanced
Findings: None

What changed in this PR

Adds a public unstable VS Code extension API for transforming supported LSP language-feature responses.

Changes:

• Implements restart-safe middleware registration, chaining, cancellation, disposal, and error recovery.
• Generates strongly typed middleware protocol definitions from the shared LSP model.
• Adds comprehensive runtime, protocol, public-type, and generation tests.

File	Description
tsc/​internal/​lsp/​lsproto/​_generate/​typeScript.mts	Generates reachable TypeScript LSP types.
tsc/​internal/​lsp/​lsproto/​_generate/​generateTypeScript.mts	Drives protocol type generation.
tsc/​internal/​lsp/​lsproto/​_generate/​generate.mts	Exposes an isolated wire-model snapshot.
tools/​scripts/​gen/​lspTypeScript.test.mts	Tests protocol generation and parsing.
tools/​scripts/​gen/​generatedFile.test.mts	Verifies both generated LSP outputs remain current.
packages/​vscode-typescript/​tsconfig.json	Resolves the shared extension API source.
packages/​vscode-typescript/​test/​tsconfig.json	Scopes extension test compilation.
packages/​vscode-typescript/​test/​lspMiddleware.test.ts	Tests middleware behavior and public types.
packages/​vscode-typescript/​test/​index.test.ts	Registers the middleware test suite.
packages/​vscode-typescript/​src/​session.ts	Owns middleware across server restarts.
packages/​vscode-typescript/​src/​lspMiddleware.ts	Implements middleware registration and execution.
packages/​vscode-typescript/​src/​extension.ts	Exposes middleware registration publicly.
packages/​vscode-typescript/​src/​contentMapperContributions.ts	Uses shared extension API types.
packages/​vscode-typescript/​src/​client.ts	Routes requests through the registry.
packages/​vscode-typescript/​package.json	Adds the local API package dependency.
packages/​typescript/​src/​vscode/​protocol.generated.ts	Provides generated middleware protocol types.
packages/​typescript/​src/​vscode/​extensionApi.ts	Defines the unstable extension API.
packages/​typescript/​package.json	Exports the new type-only API path.
package-lock.json	Records the workspace dependency.
Herebyfile.mjs	Integrates protocol type generation.

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[Ryan Cavanaugh (RyanCavanaugh)]

✅ if there's a good reason for the .. rootDir