Iceberg incremental tests

[kraftaa]

Summary

This PR adds incremental-correctness test coverage on top of the Iceberg branch, focused on failure/restart safety and duplicate-watermark paging behavior.

What’s Included

1. Duplicate watermark paging test

• Verifies composite cursor pagination (watermark + tiebreaker) does not skip rows when many records share the same watermark value.

2. State migration test

• Verifies legacy watermarks schema (without cursor_value) is migrated on open and existing progress is preserved.

3. Resume-from-progress test

• Verifies sync resumes from saved progress and writes only remaining rows.

4. Missing cursor state guard test

• Verifies cursor mode fails fast when state has watermark but missing cursor.

5. Unique watermark mode test

• Verifies incremental_column_is_unique: true works without tiebreaker and persists progress.

6. Test intent comments

• Added one-line comments above new tests for quick reviewer context.

Files Changed

• src/reader.rs
• src/state.rs
• src/sync.rs

New/Updated Tests

• read_batch_cursor_paging_handles_duplicate_watermarks
• open_migrates_legacy_watermarks_table
• sync_table_resumes_from_saved_progress
• sync_table_fails_when_cursor_state_missing_in_cursor_mode
• sync_table_supports_unique_watermark_without_tiebreaker

Validation

• cargo fmt --all
• cargo clippy --all-targets -- -D warnings
• cargo test (all passing)

Notes

• DB-backed integration-style tests are env-gated via RUSTREAM_IT_DB_URL and no-op when not set.
• No production behavior changes intended in this PR; scope is test coverage and test documentation.