fix: populate problem weight from max_score when not set

[pine919]

Description

Fixes issue where Studio's edit popup shows incorrect weight for legacy XML problems with multiple sub-problems.

Problem: When importing courses with legacy XML problems containing multiple response fields (e.g., 3 multiple-choice questions), the problem correctly displays "3 points possible" and grades properly, but clicking "Edit" in Studio shows "1 point" instead of "3 points" in the settings popup.

Root Cause: Problems imported without an explicit weight attribute don't have the field stored in block settings. The own_metadata() function only returns explicitly set fields, so Studio's edit popup shows the default weight (1) instead of the calculated value from max_score().

Solution: Modified get_block_info() to populate weight from max_score() for problem blocks without explicit weight. This only affects metadata returned for Studio editing, not stored data or grading behavior.

Impact:

• Course Authors: Studio edit popup now shows correct weight for imported legacy problems
• No changes to learner experience or grading (already working correctly)
• Backward compatible with existing problems

Supporting information

Fixes #37993

Testing instructions

1. Import a course with a legacy XML problem containing multiple sub-problems (or create one with multiple <multiplechoiceresponse> tags)
2. Navigate to the course outline in Studio
3. Verify the problem shows "X points possible" where X > 1
4. Click "Edit" on the problem
5. Verify the settings popup now shows the correct weight (X points) instead of 1 point

Unit Tests:

pytest cms/djangoapps/contentstore/xblock_storage_handlers/tests/test_problem_weight_fix.py

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