fix(lsp): use Node.js child_process on Windows to avoid Bun spawn segfault

Bun has unfixed segfault issues on Windows when spawning subprocesses
(oven-sh/bun#25798, #26026, #23043). Even upgrading to Bun v1.3.6+
does not resolve the crashes.

Instead of blocking LSP on Windows with version checks, use Node.js
child_process.spawn as fallback. This allows LSP to work on Windows
regardless of Bun version.

Changes:
- Add UnifiedProcess interface bridging Bun Subprocess and Node ChildProcess
- Use Node.js spawn on Windows, Bun spawn on other platforms
- Add CWD validation before spawn to prevent libuv null dereference
- Add binary existence pre-check on Windows with helpful error messages
- Enable shell: true for Node spawn on Windows for .cmd/.bat resolution
- Remove ineffective Bun version blocking (v1.3.5 check)
- Add tests for CWD validation and start() error handling

Closes #1047
Ref: oven-sh/bun#25798

src/tools/lsp/client.test.ts

@@ -12,7 +12,7 @@ mock.module("vscode-jsonrpc/node", () => ({
   StreamMessageWriter: function StreamMessageWriter() {},
 }))
 
-import { LSPClient } from "./client"
+import { LSPClient, validateCwd } from "./client"
 import type { ResolvedServer } from "./types"
 
 describe("LSPClient", () => {
@@ -60,4 +60,91 @@ describe("LSPClient", () => {
       }
     })
   })
+
+  describe("validateCwd", () => {
+    it("returns valid for existing directory", () => {
+      // #given
+      const dir = mkdtempSync(join(tmpdir(), "lsp-cwd-test-"))
+
+      try {
+        // #when
+        const result = validateCwd(dir)
+
+        // #then
+        expect(result.valid).toBe(true)
+        expect(result.error).toBeUndefined()
+      } finally {
+        rmSync(dir, { recursive: true, force: true })
+      }
+    })
+
+    it("returns invalid for non-existent directory", () => {
+      // #given
+      const nonExistentDir = join(tmpdir(), "lsp-cwd-nonexistent-" + Date.now())
+
+      // #when
+      const result = validateCwd(nonExistentDir)
+
+      // #then
+      expect(result.valid).toBe(false)
+      expect(result.error).toContain("Working directory does not exist")
+    })
+
+    it("returns invalid when path is a file", () => {
+      // #given
+      const dir = mkdtempSync(join(tmpdir(), "lsp-cwd-file-test-"))
+      const filePath = join(dir, "not-a-dir.txt")
+      writeFileSync(filePath, "test content")
+
+      try {
+        // #when
+        const result = validateCwd(filePath)
+
+        // #then
+        expect(result.valid).toBe(false)
+        expect(result.error).toContain("Path is not a directory")
+      } finally {
+        rmSync(dir, { recursive: true, force: true })
+      }
+    })
+  })
+
+  describe("start", () => {
+    it("throws error when working directory does not exist", async () => {
+      // #given
+      const nonExistentDir = join(tmpdir(), "lsp-test-nonexistent-" + Date.now())
+      const server: ResolvedServer = {
+        id: "typescript",
+        command: ["typescript-language-server", "--stdio"],
+        extensions: [".ts"],
+        priority: 0,
+      }
+      const client = new LSPClient(nonExistentDir, server)
+
+      // #when / #then
+      await expect(client.start()).rejects.toThrow("Working directory does not exist")
+    })
+
+    it("throws error when path is a file instead of directory", async () => {
+      // #given
+      const dir = mkdtempSync(join(tmpdir(), "lsp-client-test-"))
+      const filePath = join(dir, "not-a-dir.txt")
+      writeFileSync(filePath, "test content")
+
+      const server: ResolvedServer = {
+        id: "typescript",
+        command: ["typescript-language-server", "--stdio"],
+        extensions: [".ts"],
+        priority: 0,
+      }
+      const client = new LSPClient(filePath, server)
+
+      try {
+        // #when / #then
+        await expect(client.start()).rejects.toThrow("Path is not a directory")
+      } finally {
+        rmSync(dir, { recursive: true, force: true })
+      }
+    })
+  })
 })