Secure registry, improve tests, update packages

[millerds]

Change Description:
Update .npmrc to use a compliant registry feed
Also include some package updates.

1. Do these changes impact any npm scripts commands (in package.json)? (e.g., running 'npm run start')
   No.

2. Do these changes impact VS Code debugging options (launch.json)?
   No.

3. Do these changes impact template output? (e.g., add/remove file, update file location, update file contents)
   No.

4. Do these changes impact documentation? (e.g., a tutorial on https://docs.microsoft.com/en-us/office/dev/add-ins/overview/office-add-ins)
   No.

If you answered yes to any of these please do the following:
> Include 'Rick-Kirkham' in the review
> Make sure the README file is correct

Validation/testing performed:
Ran automated tests

[Copilot]

Pull request overview

This PR updates the repository’s npm registry configuration to use a compliant feed, refreshes several devDependencies, and tightens TypeScript/test code to improve end-to-end test reliability and failure diagnostics.

Changes:

• Point npm installs at a compliant registry feed via .npmrc (and document registry authentication).
• Improve E2E test robustness: add explicit timeouts, propagate taskpane errors back to the test runner, and add retry logic for transient #CALC! results.
• Modernize TypeScript configuration (Node16 module resolution, strict: true, additional ambient types) and adjust code to satisfy stricter typing.

Reviewed changes

Copilot reviewed 11 out of 13 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
tsconfig.json	Updates TS compiler settings (Node16 module resolution, strict mode, target es6, explicit types).
test/unit/excel.test.ts	Improves typing for the Excel mock and uses safer global assignment style.
test/end-to-end/tests.ts	Adds deterministic timeout/error handling for fetching test results; aligns result shape to {name,value}.
test/end-to-end/src/test-taskpane.ts	Adds initialization/run error reporting, retries for #CALC!, and returns captured values from readCFData.
test/end-to-end/src/test-taskpane.html	Adds lang="en" and adjusts initial visibility of #app-body.
test/end-to-end/src/test-helpers.ts	Adds typed Excel.RequestContext and introduces error-formatting helpers for test reporting.
test/end-to-end/src/debugger-websocket.ts	Removes request dependency and switches to fetch with timeouts; adds typing on websocket handlers.
src/taskpane/taskpane.ts	Adds non-null assertions for DOM elements and types the Excel request context.
src/commands/commands.ts	Avoids crashing when mailbox.item is unavailable by using optional chaining.
README.md	Documents npm registry authentication setup.
package.json	Updates several devDependencies and removes request.
.npmrc	Sets registry to the compliant feed and enables always-auth.

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