fix: Handle TCP segmentation in SLK file transitions

[as51340]

What

Fix a bug in SLK stream processing where TCP segmentation during file transitions caused CheckStreamStatus to incorrectly return NEW_FILE instead of PARTIAL. The fix adds metadata completeness validation
in CheckStreamStatus() (src/slk/streams.cpp) when a kFileSegmentMask is encountered during a file transition (i.e., remaining_file_size is set), ensuring the full file metadata (filename string + file
size uint) is present before signaling the transition.

Why

When the SLK protocol transitions between files in a multi-file transfer, the sender emits kFileSegmentMask followed by the new file's metadata (filename + size). TCP can split the segment boundary such that
the mask arrives in one packet but the metadata arrives later. Without this fix, the OpenFile path in the Reader would attempt to parse incomplete metadata and fail with "Size data missing in SLK stream!".

How

In CheckStreamStatus, when kFileSegmentMask is encountered and remaining_file_size is set (indicating a file-to-file transition, not the first file), the code now performs a two-stage check before
returning NEW_FILE:

1. String prefix check — verifies at least 9 bytes (1 marker + 8 length) are available after the mask.
2. Full metadata check — reads the string length, then verifies the entire metadata block (string prefix + string data + uint marker + uint value) fits in the remaining buffer.

If either check fails, PARTIAL is returned with the appropriate stream_size hint so the caller knows how much more data to wait for. The check is gated on remaining_file_size being set, so first-file
detection is unaffected.

Testing

Added 5 unit tests in tests/unit/slk_streams.cpp:

• FileTransitionMaskOnly — mask present but zero metadata bytes; expects PARTIAL
• FileTransitionPartialStringPrefix — fewer than 9 bytes after mask; expects PARTIAL
• FileTransitionPartialStringData — string prefix present but string body truncated; expects PARTIAL
• FileTransitionSufficientMetadata — complete metadata present; expects NEW_FILE (positive case)
• FirstFileMaskNotAffectedByCheck — no remaining_file_size; verifies the new guard doesn't regress first-file detection

Notes for reviewers

• The stream_size values returned in the PARTIAL branches are best-effort hints (pos + kSegmentMaxTotalSize when the string length is unknown, pos + needed when it is). Verify these are sufficient for
  the caller's read-more logic.
• The guard str_len <= kSegmentMaxDataSize protects against corrupted length fields — if the length is impossibly large, the code falls through to the existing NEW_FILE return, which will let the Reader
  surface a more specific error during actual parsing

[as51340]

Tracking

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• Update unit/E2E tests
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• Write a release note, including added/changed clauses
  • Fixed a bug where large file transfers between Memgraph instances (such as snapshots during replication) could fail with a "Size data missing in SLK stream!" error when TCP split the data at an unfortunate
    boundary between consecutive files. This was most likely to occur under high network load or with large snapshot files. No action is required — upgrading to this version resolves the issue automatically. #3925
  • What has changed? What does it mean for a user? What should a user do with it? [#{{PR_number}}]({{link to the PR}})
• [ Documentation PR link memgraph/documentation#XXXX ]
  • Is back linked to this development PR

[sonarqubecloud]

Quality Gate failed

Failed conditions
B Maintainability Rating on New Code (required ≥ A)

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