chore: fix miniblock selection logic error

[broccoliSpicy]

This PR tries to fix a logic error during selecting miniblock encoder.

[github-actions[bot]]

ACTION NEEDED Lance follows the Conventional Commits specification for release automation.

The PR title and description are used as the merge commit message. Please update your PR title and description to match the specification.

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[broccoliSpicy]

This is based on TPC-H testing right? Do you need <= 256? Or is < 256 ok? I thought one of the columns had a max length of 256?

in the TPC-H schema, the largest string length it has is 199 in

CREATE TABLE partsupp
(
    ps_partkey     BIGINT not null,
    ps_suppkey     BIGINT not null,
    ps_availqty    BIGINT not null,
    ps_supplycost  DOUBLE PRECISION  not null,
    ps_comment     VARCHAR(199) not null
);

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 77.11%. Comparing base (3f2faf2) to head (37fe3bf).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3107 +/- ## ========================================== - Coverage 77.13% 77.11% -0.02% ========================================== Files 240 240 Lines 80652 80643 -9 Branches 80652 80643 -9 ========================================== - Hits 62208 62188 -20 + Misses 15275 15274 -1 - Partials 3169 3181 +12 ``` | [Flag](https://app.codecov.io/gh/lancedb/lance/pull/3107/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lancedb) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/lancedb/lance/pull/3107/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lancedb) | `77.11% <100.00%> (-0.02%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=lancedb#carryforward-flags-in-the-pull-request-comment) to find out more.

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