[v3-2-test] CI: Notify open PRs that conflict on uv.lock after main merges (#65765)

[github-actions]

• CI: Notify open PRs that conflict on uv.lock after main merges

When uv.lock changes on main, open PRs that touch uv.lock frequently
end up with rebase conflicts. This adds a workflow that, on each push to
main that modifies uv.lock, scans open non-draft PRs (updated in the
last 14 days), identifies those currently reporting conflicts, and posts
(or updates, deduped via an HTML-comment marker) a single comment with
rebase-and-uv lock instructions.

The source PR that caused the change is resolved from the squash-merge
commit headline's trailing (#NNN) and linked in the notice.

Implemented as a standalone PEP 723 Python script (httpx + GitHub's
GraphQL API) so logic stays testable; workflow YAML is a thin driver that
installs uv from uv.lock and runs the script. Unit tests cover the
regex, classifier, pagination/early-exit, UNKNOWN-mergeable retry loop,
and notice posting.

• Fix scripts-tests by stubbing httpx in unit-test module

The scripts-project CI venv doesn't have httpx installed, so importing
notify_uv_lock_conflicts fails with ModuleNotFoundError: No module named 'httpx' before any test runs. httpx is a PEP 723 runtime dep of
the script; the tests mock the client and never need the real library.

Stub httpx in sys.modules with setdefault before loading the
module under test: keeps the scripts distribution's declared deps
unchanged, and leaves real httpx alone when present (e.g. in the
workspace-wide dev environment).
(cherry picked from commit 4fae29c)

Co-authored-by: Jarek Potiuk jarek@potiuk.com