test: add malformed parser coverage for migration snapshots

[emperorsixpacks]

Fixes #321

Summary

Adds comprehensive parser-level tests for malformed JSON and binary snapshots in data_migration/src/lib.rs.

Test Coverage Added (17 new tests)

Malformed JSON

• Empty bytes, random bytes, truncated JSON, schema mismatch, incompatible/future version, tampered checksum, null bytes, array root

Malformed Binary (bincode)

• Empty bytes, random bytes, truncated binary, bit-flip, incompatible version, tampered checksum, all-zero bytes

Cross-format confusion

• JSON bytes fed to binary importer
• Binary bytes fed to JSON importer

Test Results: 25 passed, 0 failed

Security Notes

• All error paths return typed MigrationError variants — no panics
• Truncation, invalid encoding, and schema mismatch are all covered