Add informational Nx affected PR checks

[isaacroldan]

WHY are these changes introduced?

We want a safe comparison path for Nx affected CI timing and coverage before replacing the existing full PR checks.

WHAT is this pull request doing?

Adds a PR-only Affected checks (informational) job that:

• checks out the PR head SHA
• fetches the PR base SHA explicitly so nx affected can compute the changed graph from a shallow checkout
• runs affected build, lint, and type-check with continue-on-error
• writes the target outcomes to the GitHub step summary

The existing full required PR jobs remain unchanged and authoritative. The affected job is intentionally non-blocking and does not run for merge_group.

How to test your changes?

• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/tests-pr.yml"); puts "YAML OK"'
• git diff --check

Full CI was not run locally.

[isaacroldan]

• Add informational Nx affected PR checks #7812 👈 (View in Graphite)
• main

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[isaacroldan]

Closing this one: we do not need the extra informational affected-check job right now.