test: port grades/tests/integration/test_problems.py coverage (grades pipeline + ProblemBlock)

Context

As part of the Slash-n-Burn Built-In XBlocks epic (openedx-platform#37819), the file `grades/tests/integration/test_problems.py` was deleted from openedx-platform in PR #38751.

This test file covered grades-pipeline integration with ProblemBlock, which now lives in xblocks-contrib. The coverage was not ported before deletion.

What needs to be ported

`grades/tests/integration/test_problems.py` covered:

• Grades pipeline integration — full grade calculation flow with ProblemBlock as the leaf component
• Score update propagation — `SCORE_PUBLISHED` signal handling through the grades subsystem
• Problem submission → grade — end-to-end: student submits answer → score stored → course grade updated
• Weighted grading — subsection and course-level grade aggregation with ProblemBlock scores
• Graded vs ungraded — behaviour differences when a ProblemBlock is inside a graded vs ungraded subsection

This integration test lives in the grades subsystem today but its primary subject — ProblemBlock correctness under grading — should have a home in xblocks-contrib.

Acceptance Criteria

• Port equivalent integration tests into xblocks-contrib (may require a lightweight grades-stub or rely on openedx-platform test infrastructure via tox)
• Tests should exercise ProblemBlock's grading contract without depending on xmodule internals
• CI must remain green

Related

• openedx-platform PR: #38751
• Epic: openedx-platform#37819

Start by reading the deleted grades/tests/integration/test_problems.py coverage and the linked openedx-platform PR #38751. Then inspect the ProblemBlock implementation in xblocks-contrib and determine whether a lightweight grades stub or tox-based openedx-platform infrastructure is appropriate. Done means equivalent tests cover grading flow, score propagation, submissions, weighted aggregation, and graded versus ungraded subsections while CI stays green.