Fix incorrect column number in funcTupleDesc initialization.

[zhangyue-hashdata]

In process_sample_rows(), when initializing funcTupleDesc for table columns, the column number should start from NUM_SAMPLE_FIXED_COLS + 1 (5) instead of 4.

The first 4 columns (1-4) are reserved for fixed columns:

• Column 1: totalrows (FLOAT8OID)
• Column 2: totaldeadrows (FLOAT8OID)
• Column 3: oversized_cols_length (FLOAT8ARRAYOID)
• Column 4: NDV array (FLOAT8ARRAYOID)

Table columns should start from column 5, so the correct formula is: (AttrNumber) NUM_SAMPLE_FIXED_COLS + 1 + index

This bug is harmless because funcTupleDesc's column type information is not actually used in subsequent processing - only the column count (natts) is used. The actual type information is obtained dynamically via lookup_rowtype_tupdesc(). However, it's still worth fixing for code correctness and maintainability.

Fixes #ISSUE_Number

What does this PR do?

Type of Change

• [ ] Bug fix (non-breaking change)
• [ ] New feature (non-breaking change)
• [ ] Breaking change (fix or feature with breaking changes)
• [ ] Documentation update

Breaking Changes

Test Plan

• [ ] Unit tests added/updated
• [ ] Integration tests added/updated
• [ ] Passed make installcheck
• [ ] Passed make -C src/test installcheck-cbdb-parallel

Impact

Performance:

User-facing changes:

Dependencies:

Checklist

• [ ] Followed contribution guide
• [ ] Added/updated documentation
• [ ] Reviewed code for security implications
• [ ] Requested review from cloudberry committers

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