fix: part 4 #12432 — batch mybooks book lookups

[CipherZ3r0]

fix: part 4 #12432
follow after #13584

Technical

Batch initial book lookups and redirect resolution in mybooks.py using get_many() to eliminate the per-loan N+1 lookup pattern while preserving the existing redirect behavior.

Testing

• 32/32 tests pass
• Pre-commit hooks pass
• Added coverage for missing keys, duplicates, redirects, multi-hop redirects, and mixed loan scenarios
• Added integration coverage for batching behavior

Performance

Replaces per-loan get() calls with batched get_many() requests, including redirect resolution.

openlibrary/tests/fastapi/test_mybooks_integration.py file is generated by AI

Stakeholders

@RayBB