Fix bulk_update raising syntax error for ForeignKey fields

[Br1an67]

Description

When bulk_update was called with a ForeignKey field name (e.g., fields=['post']), the generated SQL used the Python object repr (e.g., <Post>) instead of the FK column value, causing a syntax error like near "<": syntax error.

The fix resolves FK fields to their source_field column name and extracts the related object's PK value, matching the behavior of the regular UpdateQuery.

Motivation and Context

Fixes #1335

Using fields=['post_id'] worked as a workaround, but fields=['post'] (the FK field name) should also work, as it does for regular .update() queries.

How Has This Been Tested?

Added test_bulk_update_foreign_key that:

1. Creates events linked to tournament1
2. Reassigns them to tournament2
3. Calls bulk_update(events, fields=['tournament'])
4. Verifies the FK was correctly updated in the database

All 8 existing bulk_update tests continue to pass.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codspeed-hq]

Merging this PR will not alter performance

✅ 24 untouched benchmarks

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_{Comparing Br1an67:fix/bulk-update-fk (15da693) with develop (f61f641)}

[abondar]

@Br1an67 Also check linter here, pls