Re-add removed main changes

[gztensor]

Description

Adds back

• migration migrate_pending_emissions_including_null_stake (which is not executed) and its test
• Two swap_hotkey tests:
  • test_hotkey_swap_stake_delta
  • test_swap_hotkey_with_pending_emissions

Related Issue(s)

n/a

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Other (please describe): resolving old merge

Breaking Change

n/a

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have run cargo fmt and cargo clippy to ensure my code is formatted and linted correctly
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Screenshots (if applicable)

n/a

Additional Notes

n/a