fix: Add ESLint rules for type safety and fix violations (#46)

* Add ESLint rules for type safety and fix violations

- Add @typescript-eslint/no-floating-promises: error
- Add @typescript-eslint/no-non-null-assertion: error
- Add @typescript-eslint/prefer-nullish-coalescing: error
- Change @typescript-eslint/no-explicit-any from off to error

Fixes:
- Fix floating promise in NotebookPicker constructor by adding .catch()
- Fix non-null assertion in NotebookPicker.onAfterAttach with null check
- Fix prefer-nullish-coalescing in handler.ts by using ?? instead of ||
- Add inline eslint-disable comments for legitimate any usage in handler.ts

* Add test for error handling in NotebookPicker constructor

Improves test coverage for the .catch() error handler added to handle
promise rejections in the constructor.

* Add comprehensive test coverage for NotebookPicker

- Add test for null model in handleChange
- Add test for invalid metadata in handleChange
- Add test for onAfterAttach without parent
- Import Message type for test

Coverage improved from 84.09% to 97.72% for NotebookPicker.tsx

* Merge test coverage from PR #45

Added comprehensive test suites for core transformation functions:
- convert-deepnote-block-to-jupyter-cell.spec.ts (13 tests)
- transform-deepnote-yaml-to-notebook-content.spec.ts (11 tests)

Overall coverage improved from 31.03% to 57.24%
Both transformation files now have 100% line coverage

* fix: remove mocks, test real implementation

* chore: format

* chore: remove out of scope tests

* chore: remove out of scope test

* refactor: fix test and format

* chore: remove out of scope test

* fix: address violations

* refactor: improve test

* refactor: TypeError

* fix: drop unnecessary property

* refactor: simplify promise

* chore: improve "dirty" test

---------

Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
Co-authored-by: dinohamzic <dino@subtlebits.com>

Author: 3 people

jest.config.js

@@ -4,13 +4,15 @@ const esModules = [
   '@codemirror',
   '@jupyter',
   '@microsoft',
+  '@deepnote',
   'exenv-es6',
   'lib0',
   'nanoid',
   'vscode-ws-jsonrpc',
   'y-protocols',
   'y-websocket',
-  'yjs'
+  'yjs',
+  'yaml'
 ].join('|');
 
 const baseConfig = jestJupyterLab(__dirname);

package.json

@@ -161,9 +161,12 @@
                     "args": "none"
                 }
             ],
-            "@typescript-eslint/no-explicit-any": "off",
+            "@typescript-eslint/no-explicit-any": "error",
             "@typescript-eslint/no-namespace": "off",
             "@typescript-eslint/no-use-before-define": "off",
+            "@typescript-eslint/no-floating-promises": "error",
+            "@typescript-eslint/no-non-null-assertion": "error",
+            "@typescript-eslint/prefer-nullish-coalescing": "error",
             "@typescript-eslint/quotes": [
                 "error",
                 "single",