fix(storage): read CSR length via Column in getDegreeForOffset

[adsharma]

Fixes #1292.

Root cause: RelTable::getDegreeForOffset read the persistent CSR length directly from the ChunkedCSRNodeGroup's ColumnChunk (ChunkedCSRHeader::getCSRLength → ColumnChunk::getValue). Those header chunks are ResidencyState::ON_DISK, so their in-memory buffers are zero-size/not positionally valid — in release builds this reads out-of-bounds/uninitialized memory, producing garbage degrees (~1e14) or 0 for valid offsets. It also bypassed the scan-pin/retained-snapshot routing.

Fix:

• Pin via captureScanPin(transaction) and scan the single length value through its Column (same pattern as forEachDegree/CountRelTable).
• Read committed in-memory degrees from pin.index.
• Count uncommitted local rels even when the node group has no committed CSR group (matches forEachDegree's local-only-group handling).

Tests: extended RelDegreeTest.MatchesCypherAcrossNodeGroupsAndStorageStates to assert getDegreeForOffset == degree for every expected entry across checkpointed, committed in-memory, and uncommitted states.