fix: sparse SINDI build validation

[sparknack]

issue: #1555

• Reject unsupported explicit sparse inverted_index_algo values during build
• Fix empty SINDI index POSTING_LISTS section size accounting

[mergify]

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[alexanderguzhva]

@sparknack should algo be converted to upper-case before the check?

[sparknack]

Done. The explicit inverted_index_algo is normalized to upper-case before validation, and CreateIndexImpl now uses the same normalized value when selecting/setting the build algorithm. I also changed the regression test to build a non-SINDI index with lower-case daat_maxscore so we cover both validation and actual index creation.

[alexanderguzhva]

@sparknack looks good overall, waiting for the reply on comments

[alexanderguzhva]

/lgtm
/approve

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[alexanderguzhva]

/kind improvement
issue: #1555