[Bigtable] Fix Bigtable segment truncation when open end key is startKey + null byte (#39842)

[aa-dit-yuh]

Fixes #39842

Problem

When reading from Cloud Bigtable using Apache Beam, reads are processed in buffered segments. When a segment hits buffer memory limits or row limits during a read, Beam pauses the stream and calls truncateRequest to construct a new ReadRowsRequest for the remainder of the key range starting from (start_key_open: lastKey).
For key ranges where the open end key is the start key with an appended null byte (e.g., [K, K\0), common in single-row scans, prefix scans, or dynamic work rebalancing splits), EndPoint.compareTo compares "K\0" to "K", evaluating to endCmp > 0. Because endCmp > 0, truncateRequest did not skip the range after reading key K, and generated a request with (start_key_open: K, end_key_open: K\0).
Cloud Bigtable server validates start_key_open as K\0 and strictly enforces start_key < end_key. Because K\0 < K\0 is false, Bigtable server rejects the request with:
io.grpc.StatusRuntimeException: INVALID_ARGUMENT: Error in field 'row_ranges' : Error in element #0 : start_key must be less than end_key

Solution

In BigtableServiceImpl.BigtableSegmentReaderImpl.truncateRequest, check if rowRange.getEndKeyCase() == END_KEY_OPEN and skip the range if lastKey + "\0" >= rowRange.getEndKeyOpen().

Tests

• Added unit test testReadRangeWithNullByteEndKeyAtByteLimit in BigtableServiceImplTest.java verifying that ranges with an open null-byte suffix end key are properly recognized as exhausted during truncation without generating invalid requests.

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[mutianf]

I noticed and fixed a similar issue with reconstructing row key ranges before in the reading large rows logic https://screenshot-v2.corp.google.com/7dli6t427bceg. Maybe it's worth using RowSetUtil.erase to handle this here. But this fix in beam looks correct.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 57.10%. Comparing base (a26ecfd) to head (c582a81).
⚠️ Report is 12 commits behind head on master.

Files with missing lines	Patch %	Lines
.../beam/sdk/io/gcp/bigtable/BigtableServiceImpl.java	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #39843   +/-   ##
=========================================
  Coverage     57.09%   57.10%           
- Complexity     3639     3654   +15     
=========================================
  Files          1191     1191           
  Lines        192556   192587   +31     
  Branches       3816     3831   +15     
=========================================
+ Hits         109946   109980   +34     
+ Misses        79042    79035    -7     
- Partials       3568     3572    +4     

Flag	Coverage Δ
java	72.14% <80.00%> (+0.04%)	⬆️

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R: @Abacn for label java.
R: @mutianf for label bigtable.

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