feat: pending_pr marker, so tests skip until their source PR merges

[saqibmanan]

Adds a pending_pr("Repo#N") marker. A test that carries it is skipped until GitHub reports that source PR merged, and after that it runs normally. This lets pr-test-sync write coverage for open dev PRs older than 72h now, so the tests are reviewed and ready when the PR lands, with no CI noise before then.

• pytest.ini: registers the marker (--strict-markers safe).
• conftest.py: pytest_collection_modifyitems looks up GET /repos/<owner>/<repo>/pulls/<N>, once per ref per session (stdlib only, no new dependency). If the PR isn't merged, or its state can't be read, the test skips with the reason shown.
• Workflows: pass GH_PR_TOKEN (the job token) so the lookups are authenticated. A test with no pending_pr marker makes no API calls.

Local check. The ungated test ran, and the gated one skipped:

SKIPPED [1] pending_pr DataSpaceFrontend#470: not merged yet
SKIPPED [1] pending_pr CivicDataLab/ParakhAI-Backend#106: could not read PR state (HTTP Error 404: Not Found)
1 passed, 2 skipped

(DataSpaceFrontend#476 is merged, so its test ran. #470 is open. The 404 is a private repo read without a token.)

Needs a secret: ParakhAI-Backend is private, so add GH_PR_TOKEN: a fine-grained PAT with Pull requests: read on that repo. Without it, tests pending on a backend PR skip with the 404 reason even after the merge. run-smoke.yml called from ParakhAI-frontend CD will not get this secret either.