Add comprehensive unit tests for migration functions

[Achieve3318]

• Add 6 tests for migrate_delete_subnet_3 (subnet deletion, storage cleanup)
• Add 7 tests for migrate_to_v1_separate_emission (format translation)
• Add 4 tests for migrate_to_v2_fixed_total_stake (version gating, docs)
• Add 7 tests for migrate_total_issuance (calculation, error handling)

Resolves all TODO comments requesting unit tests and error handling in migration files. Total: 733 lines, 24 new tests.

Description

Related Issue(s)

• Closes #[issue number]

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):

Breaking Change

If this PR introduces a breaking change, please provide a detailed description of the impact and the migration path for existing applications.

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 ./scripts/fix_rust.sh 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)

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Additional Notes

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