Trac ticket number

ticket-29291

Branch description

This PR fixes a long-standing issue where using a negated Q() object inside a When() expression would result in incorrect SQL due to improper join handling across nullable relationships.

Previously, annotations like:

annotate(
    flag=Case(
        When(~Q(related_model__field=True), then=Value(True)),
        default=Value(False),
    )
)

would generate broken or incomplete SQL when traversing nullable FKs. This patch updates When.as_sql() to delegate condition compilation to the query's internal _add_q() logic using query.build_filter(...), ensuring consistent handling of negation, joins, and subquery pushdowns -- just like .filter().

This patch includes:

• A new test class documenting expected behavior and edge cases
• Full test coverage comparing annotated vs filtered queries.
• No changes to behavior for non-Q conditions

Checklist

• This PR targets the main branch.
• The commit message is written in past tense, mentions the ticket number, and ends with a period.
• I have checked the "Has patch" ticket flag in the Trac system.
• I have added or updated relevant tests.
• I have added or updated relevant docs, including release notes if applicable.
• I have attached screenshots in both light and dark modes for any UI changes.