[YSQL] Fixes unique index consistency bug when yb_enable_inplace_index_update is on

[austenLacy]

yb_enable_inplace_index_update incorrectly treats primary-key updates on unique secondary indexes as safe for in-place index-row updates. This is false when the unique index uses NULLs-distinct semantics and any unique-index key column is NULL, because the hidden ybuniqueidxkeysuffix is part of the index row key and is derived from the base row ybctid. A base table PK update changes the base row ybctid, so the hidden unique suffix changes too.

Reproducible example:

SET yb_enable_inplace_index_update = on;

CREATE TABLE idx_test (
 c1 bigint,
 id bigint,
 foo smallint DEFAULT 0 NOT NULL,
 some_hash bytea,
 is_not_deleted boolean,
 PRIMARY KEY ((c1) HASH, id ASC, foo ASC)
);

CREATE UNIQUE INDEX idx_test_uq
ON idx_test USING lsm (c1 HASH, some_hash ASC)
WHERE is_not_deleted;

INSERT INTO idx_test (c1, id, foo, some_hash, is_not_deleted)
VALUES (1, 1, 0, NULL, true);

UPDATE idx_test SET foo = 1
WHERE c1 = 1 AND id = 1 AND foo = 0;

SELECT yb_index_check('idx_test_uq'::regclass);

Consistency check immediately fails

yugabyte=# SELECT yb_index_check('idx_test_uq'::regclass);
ERROR:  ybuniqueidxkeysuffix and ybbasectid mismatch
DETAIL:  index: 'idx_test_uq', ybbasectid: '\x47eda949800000000000000121498000000000000001488000000121'

The fix is to essentially force the DELETE+INSERT for this scenario.

With yb_enable_inplace_index_update = off the issue is not reproducible with the above example.

[gemini-code-assist]

Code Review

This pull request ensures that primary key updates on unique indexes with NULL key columns are modeled as DELETE and INSERT operations rather than in-place updates, because the index row key suffix depends on the base table's ybctid. Feedback on the changes suggests switching to a short-lived per-tuple memory context in the newly introduced YbUniqueIndexKeyHasNull function to prevent potential memory leaks during large bulk updates when calling FormIndexDatum.

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[austenLacy]

trigger jenkins