Upgrades eslint to v9

[milanholemans]

Closes #6350

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After upgrading to the latest Eslint version, a lot of extra rules were activated, which all made sense. This means I had to make quite a few changes to files violating these rules.

[Copilot]

Pull Request Overview

This PR upgrades ESLint from v8 to v9, which required extensive code changes to comply with newly activated rules. The upgrade modernizes the linting configuration by migrating from the legacy .eslintrc.cjs format to the new flat config system in eslint.config.mjs.

• Migrate to ESLint v9 flat configuration system
• Fix violations from newly activated ESLint rules across the codebase
• Update TypeScript and Mocha ESLint plugins to compatible versions

Reviewed Changes

Copilot reviewed 161 out of 163 changed files in this pull request and generated 8 comments.

File	Description
package.json	Updated ESLint and related dependencies to v9 compatible versions
eslint.config.mjs	New flat configuration replacing the old .eslintrc.cjs format
.eslintrc.cjs	Removed legacy ESLint configuration file
Various TS files	Fixed ESLint rule violations including function return types, unused catch variables, regex escaping, and code formatting

Files not reviewed (1)

• npm-shrinkwrap.json: Language not supported

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