Scope release-file bulk deletion to the requested release

[e-q]

Description

• Build the delete_by_release queryset from the requested release_id
  instead of the regular filtered list queryset.
• Extend the existing v2 API test so an additional valid release-file filter
  does not leave files for the requested release behind.
• Return 400 for missing, blank, or malformed release values.
• Assert files for a different release remain after the bulk delete.

Closes

• Closes Bug: release-file bulk deletion can leave files behind #3012

Testing

• Focused v2 delete_by_release regression test, including extra filters and
  invalid release values: passed.
• Focused Ruff lint and format checks on touched files: passed.
• Whitespace check: passed.