[core] Fix lookup merge buffer serialization for projected reads

[reswqa]

Purpose

Fix lookup merge buffer serialization when reading with a projected schema.

LookupMergeFunction.Factory currently creates the candidate buffer using the full valueType, even when the input rows follow a projected readType. Once the number of candidates for a key exceeds lookup.merge-records-threshold, the buffer switches to binary serialization. The mismatch between the serializer schema and the actual row layout can cause read failures when fields are pruned or reordered, including nested projections.

This PR uses readType to derive the buffer schema when it is provided, preserving the projected field order and nested structure. Nullable fields from the internal valueType are retained by matching field IDs. This preserves the delete-record handling introduced in #9978, where omitted values may be null even if the corresponding fields are declared NOT NULL in the read schema.

The original readType is still passed unchanged to the wrapped merge function. Calls without a readType continue to use valueType.

No public API or table file-format changes.

Tests

Added regression coverage for:

• Pruned and reordered fields during lookup merge buffering.
• Delete records containing null values for fields declared NOT NULL in the read schema.
• Nested field projections.
• Projected batch reads across two data files with lookup changelog enabled and lookup.merge-records-threshold set to 1.

Validated the following test classes with JDK 8:

• LookupMergeFunctionTest
• LookupMergeFunctionUnitTest
• LookupChangelogMergeFunctionWrapperTest
• LookupMergeTreeCompactRewriterTest
• PrimaryKeySimpleTableTest

all tests passed, with no failures, errors, or skips.

[JunRuiLee]

LGTM, pending green CI.