fix(editor): highlight Solidity files (#20928)

Map .sol to Monaco's built-in 'sol' language id. The grammar already
ships with monaco-editor; only the extension lookup was missing, so
.sol fell through to plaintext.

Closes #13835

Co-authored-by: Neil <neil@stably.ai>

src/renderer/src/lib/language-detect.test.ts

@@ -112,6 +112,11 @@ describe('detectLanguage', () => {
     expect(detectLanguage('C:\\theme\\snippets\\CART.LIQUID')).toBe('liquid')
   })
 
+  it('maps .sol files to the Monaco built-in sol language id, not the solidity alias', () => {
+    expect(detectLanguage('contracts/Vault.sol')).toBe('sol')
+    expect(detectLanguage('C:\\repo\\contracts\\TOKEN.SOL')).toBe('sol')
+  })
+
   it('maps Salesforce Apex sources to the apex language id (case-insensitive)', () => {
     expect(detectLanguage('force-app/main/default/classes/AccountService.cls')).toBe('apex')
     expect(detectLanguage('force-app/main/default/triggers/AccountTrigger.trigger')).toBe('apex')

src/renderer/src/lib/language-detect.ts

@@ -93,6 +93,8 @@ const EXT_TO_LANGUAGE: Record<string, string> = {
   '.hrl': 'erlang',
   '.hs': 'haskell',
   '.clj': 'clojure',
+  // Why: Monaco registers Solidity under the id 'sol'; 'solidity' is only an alias.
+  '.sol': 'sol',
   '.vue': 'vue',
   '.svelte': 'svelte',
   '.astro': 'astro',

src/renderer/src/lib/solidity-language-id-registration.test.ts

@@ -0,0 +1,20 @@
+import { readFileSync } from 'node:fs'
+import { createRequire } from 'node:module'
+import { describe, expect, it } from 'vitest'
+import { detectLanguage } from './language-detect'
+
+// Why: Monaco registers Solidity as id 'sol' with 'solidity' only an alias, and
+// mapping an alias silently renders plaintext. Pin against the installed package
+// so an upstream id change fails here instead of in the editor.
+describe('solidity language id', () => {
+  it('matches the id monaco-editor registers for .sol', () => {
+    const require = createRequire(import.meta.url)
+    const source = readFileSync(
+      require.resolve('monaco-editor/esm/vs/basic-languages/solidity/solidity.contribution.js'),
+      'utf8'
+    )
+    const registeredId = /id:\s*["']([^"']+)["']/.exec(source)?.[1]
+    expect(registeredId).toBeDefined()
+    expect(detectLanguage('contracts/Vault.sol')).toBe(registeredId)
+  })
+})