fix(lsp): improve error messages when LSP server is not installed (#305)

Previously, when an LSP server was configured but not installed, the error
message said "No LSP server configured" which was misleading. Now the
error message distinguishes between:

1. Server not configured at all
2. Server configured but not installed (with installation hints)

The new error messages include:
- Clear indication of whether server is configured vs installed
- Installation commands for each built-in server
- Supported file extensions
- Configuration examples for custom servers

Fixes #304

Co-authored-by: sisyphus-dev-ai <sisyphus-dev-ai@users.noreply.github.com>

src/tools/lsp/utils.ts

@@ -17,6 +17,7 @@ import type {
   TextEdit,
   CodeAction,
   Command,
+  ServerLookupResult,
 } from "./types"
 
 export function findWorkspaceRoot(filePath: string): string {
@@ -40,15 +41,51 @@ export function findWorkspaceRoot(filePath: string): string {
   return require("path").dirname(resolve(filePath))
 }
 
+export function formatServerLookupError(result: Exclude<ServerLookupResult, { status: "found" }>): string {
+  if (result.status === "not_installed") {
+    const { server, installHint } = result
+    return [
+      `LSP server '${server.id}' is configured but NOT INSTALLED.`,
+      ``,
+      `Command not found: ${server.command[0]}`,
+      ``,
+      `To install:`,
+      `  ${installHint}`,
+      ``,
+      `Supported extensions: ${server.extensions.join(", ")}`,
+      ``,
+      `After installation, the server will be available automatically.`,
+      `Run 'lsp_servers' tool to verify installation status.`,
+    ].join("\n")
+  }
+
+  return [
+    `No LSP server configured for extension: ${result.extension}`,
+    ``,
+    `Available servers: ${result.availableServers.slice(0, 10).join(", ")}${result.availableServers.length > 10 ? "..." : ""}`,
+    ``,
+    `To add a custom server, configure 'lsp' in oh-my-opencode.json:`,
+    `  {`,
+    `    "lsp": {`,
+    `      "my-server": {`,
+    `        "command": ["my-lsp", "--stdio"],`,
+    `        "extensions": ["${result.extension}"]`,
+    `      }`,
+    `    }`,
+    `  }`,
+  ].join("\n")
+}
+
 export async function withLspClient<T>(filePath: string, fn: (client: LSPClient) => Promise<T>): Promise<T> {
   const absPath = resolve(filePath)
   const ext = extname(absPath)
-  const server = findServerForExtension(ext)
+  const result = findServerForExtension(ext)
 
-  if (!server) {
-    throw new Error(`No LSP server configured for extension: ${ext}`)
+  if (result.status !== "found") {
+    throw new Error(formatServerLookupError(result))
   }
 
+  const server = result.server
   const root = findWorkspaceRoot(absPath)
   const client = await lspManager.getClient(root, server)