Dispatch Checks + Use new mypy check in CI

[Scott Beddall (scbedd)]

This PR:

• Adds new script entrypoint eng/scripts/dispatch_checks.py which will be very similar to dispatch_tox, but simplified and meant to supplant only the features we really need.
• Swaps our mypy utilization to use our new checks system
• Filter out checks that are disabled BEFORE invoking them and possibly spinning an isolated venv
• Ensure we pass the --isolate argument to each call within CI
• Test negative case (add a mypy failure) See this build on a branch with an introduced mypy error

todo:

• Errors should be a bit more obvious in the output. Right now I can't tell where the failure is I lied you definitely can

What this PR does NOT do is enforce the verbosity for the shellouts that are being done. There will be more followups on that in #42862

jenny (@JennyPng)

[Copilot]

Pull Request Overview

This PR introduces a new dispatch checks system by adding a new script eng/scripts/dispatch_checks.py that provides a simplified alternative to the existing dispatch_tox functionality. The script enables concurrent execution of multiple checks across packages using asyncio for improved performance.

Key changes:

• Implements a new async-based check dispatch system with configurable concurrency
• Provides structured output capture and summarization for check results
• Includes comprehensive argument parsing for CI integration and developer workflows