[python] Add SnapshotManager batch lookahead with get_snapshots_batch and find_next_scannable

[tub]

Summary

• Adds SnapshotManager.get_snapshots_batch(snapshot_ids, max_workers=4): batch-checks existence of a list of snapshot files via file_io.exists_batch(), then fetches the existing ones in parallel using ThreadPoolExecutor, returning {id: Snapshot|None}
• Adds SnapshotManager.find_next_scannable(start_id, should_scan, lookahead_size=10, max_workers=4): looks ahead lookahead_size snapshot IDs, fetches them in one batch, and returns (snapshot, next_id, skipped_count) for the first snapshot passing should_scan

These two methods are the performance foundation for AsyncStreamingTableScan (coming in 2c):

• exists_batch replaces N serial file existence checks with a single round trip — important on object stores where per-call latency is significant
• Lookahead allows the scan loop to skip non-scannable commits (e.g. COMPACT, OVERWRITE) without a separate round trip per snapshot
• skipped_count in the return value enables the prefetch path in the streaming scan to submit the next lookahead fetch to a background thread while the consumer processes the current plan

Stack context

This is part of a stack of PRs adding streaming read support to paimon-python. Each PR is independently reviewable with a narrow scope:

PR	Branch	Content
#7417	python-streaming-2a-changelog-producer	ChangelogProducer enum + config option
This PR	python-streaming-2b-snapshot-lookahead	SnapshotManager.get_snapshots_batch() + find_next_scannable()
Next	python-streaming-2c-scan-and-builder	AsyncStreamingTableScan, StreamReadBuilder, Table.new_stream_read_builder()
Next	python-streaming-2d-consumer	Consumer ID integration into scan/builder
Next	python-streaming-2e-acceptance-docs	IncrementalDiffScanner acceptance tests + streaming docs

Tracking issue: #7152

Test plan

• cd paimon-python && python -m pytest pypaimon/tests/snapshot_manager_test.py -v
• cd paimon-python && flake8 pypaimon/snapshot/snapshot_manager.py pypaimon/tests/snapshot_manager_test.py

🤖 Generated with Claude Code

[JingsongLi]

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