fix: add cursor iteration support to psycopg2 MockCursor for django.contrib.postgres

[jy-tan]

Summary

Fixes a bug where Django apps with django.contrib.postgres in INSTALLED_APPS would crash during startup in REPLAY mode with TypeError: 'MockCursor' object is not iterable.

Django's postgres extension iterates over cursor results to register type handlers (hstore, citext, etc.):

# django/contrib/postgres/signals.py
for row in cursor:
    ...

The MockCursor class was missing __iter__ and __next__ methods required for this iteration pattern.

Changes

• Add __iter__ and __next__ methods to MockCursor class for REPLAY mode cursor iteration
• Add __iter__ and __next__ methods to InstrumentedCursor class for RECORD mode cursor iteration (uses stored _tusk_rows after _finalize_query_span captures rows)
• Add django.contrib.postgres to the django-postgres stack test to validate the fix
• Add /db/cursor-iteration test endpoint that exercises for row in cursor syntax

[cubic-dev-ai]

No issues found across 5 files