test: add mock-based orchestration tests

[bitalizer]

Summary

• Add 17 mock-based tests for DatabaseMigrator and TableMigrator using mock Extractor/Inserter trait implementations
• Validates orchestration logic (table filtering, whitelist, batching, fail-fast, constraints, snake_case) without real database connections
• Builds on the trait abstractions from PR refactor: trait abstractions for testable DB layer #19

Tests added

DatabaseMigrator (11 tests):

• Single/multiple table migration
• Whitelist filtering and missing whitelist match
• No tables error
• Packet size validation
• Fail-fast on table error with cancellation
• Table already has rows rejection
• Existing empty table reuse
• Snake case formatting
• Constraints creation flow

TableMigrator (6 tests):

• Empty table, single batch, multiple batches
• Schema fetch error, mapping not found error
• Table creation failure

Test plan

• All 130 tests pass (113 existing + 17 new)
• cargo clippy -- -D warnings clean
• cargo fmt --check clean