Update eslint

[pontemonti]

Meant to address https://github.com/microsoft/Agent365-nodejs/security/dependabot/11

We were using an old version of eslint with an old config file format.
This PR updates to the latest eslint version, creates a "global" eslint config file, removes old (outdated, old format) eslint config files, and addresses some eslint errors that came up during this upgrade.
Also removed some npm package-lock.json files that should no longer be used, and as a result had to update the github workflow to use pnpm as package manager and cache based on pnpm-lock.yaml instead of package-lock.json.

[Copilot]

Pull Request Overview

This pull request updates ESLint from version 8 to version 9, migrating from the legacy .eslintrc.json configuration format to the new flat config format (eslint.config.mjs). The update also addresses ESLint errors that surfaced with the new version.

Key Changes:

• Updated ESLint and TypeScript ESLint packages from v6/v8 to v8/v9
• Created a global eslint.config.mjs file with flat config format
• Removed individual .eslintrc.json files from subdirectories
• Fixed code to comply with new ESLint rules (prefixing unused variables with _, adding eslint-disable comments for intentional any types)
• Removed package-lock.json files in favor of pnpm-lock.yaml
• Updated GitHub workflow to use pnpm

Reviewed Changes

Copilot reviewed 38 out of 41 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
eslint.config.mjs	New global ESLint flat config file with recommended rules
pnpm-workspace.yaml	Updated ESLint and TypeScript dependencies to latest versions
pnpm-lock.yaml	Updated lock file with new ESLint v9 dependencies
tests/package.json, tests-agent/*/package.json	Updated dev dependencies to ESLint v9 and TypeScript ESLint v8
packages/*/package.json	Updated dev dependencies across all packages consistently
tests/.eslintrc.json, tests-agent/*/src/.eslintrc.json, packages/*/src/.eslintrc.json	Removed old ESLint config files
tests-agent/basic-agent-sdk-sample/package-lock.json	Removed npm lock file (now using pnpm)
TypeScript source files	Fixed ESLint violations by prefixing unused variables with _ and adding eslint-disable comments for intentional any usage

Files not reviewed (2)

• pnpm-lock.yaml: Language not supported
• tests-agent/basic-agent-sdk-sample/package-lock.json: Language not supported