Enable strict TypeScript, add errorMessage helper (#37292)

Enable full TypeScript `strict` mode and fix issues discovered during
this refactor. Introduced a `errorMessage` helper function to cleanly
extract a error messages from the `unknown` type.

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (claude-opus-4-7) <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

tsconfig.json

@@ -35,8 +35,6 @@
     "skipLibCheck": true,
     "sourceMap": true,
     "strict": true,
-    "strictPropertyInitialization": false,
-    "useUnknownInCatchVariables": false,
     "stripInternal": true,
     "verbatimModuleSyntax": true,
     "types": [