Fix: SQLAlchemy DetachedInstanceError in creative loading

[danf-newton]

Problem:
When creating media buys, creative objects were being accessed after the database session closed, causing DetachedInstanceError when trying to read attributes like creative.data, creative.format, creative.name.

Root Cause:
SQLAlchemy lazy-loads attributes. When creative objects left the session context, their attributes became inaccessible. Subsequent access in loops triggered DetachedInstanceError.

Solution:
Eagerly load all required attributes into a plain Python dict immediately after querying, while still in session. Store dict instead of ORM object for reads. Keep ORM object reference only for database updates.

Changes:

• Lines 2794-2807: Create creatives_by_id dict with eager-loaded data
• Lines 2823+: Use creative_dict values instead of ORM attributes
• Line 2891: Fix asset 'id' field (Mock adapter compatibility)

Impact:

• Fixes campaign creation failures during creative assignment
• No performance impact (same number of queries)
• Prevents session detachment errors in all code paths

Testing:
Verified with Newton integration test - campaign creation now succeeds without DetachedInstanceError.

Resolves the 'creative not being properly bound to a session' error.

[bokelley]

@danf-newton sorry this didn't get merged quickly. Do you have a test case we could add here to 1) see if this is still breaking and 2) make sure the fix doesn't regress?

[github-actions]

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