fix(index): sort JSON-path values once after extraction, not the raw column

[wjones127]

Problem

A JSON-path scalar index (target_index_type=btree) returns wrong results when the
indexed path holds float64 values that are not exactly representable (e.g. 40.1,
-3.2): ranges come back empty and equality misses rows.

Fixes #7485.

Root cause

The btree trainer requires its input sorted by value (page min/max are taken from the
first/last row of each page). scan_training_data sorts by the raw JSON column when
the target requests TrainingOrdering::Values, but JsonIndexPlugin extracts a
different value (the value at path) from that column downstream, so the extracted
stream is not actually sorted by value. Silently corrupted page stats then cause range
and equality lookups to skip pages that hold matching rows.

Relationship to #7493, #7771, #7819

All three open PRs diagnose this correctly and fix it by adding a second SortExec
after JSON extraction, on top of the raw-column sort the scanner already performs. That
raw-column sort is wasted work — it sorts a key nothing downstream uses.

This PR instead has JsonTrainingRequest::criteria() override the target's
TrainingOrdering::Values to TrainingOrdering::None when delegating to the scanner
(the scanner's sort can't help the extracted value anyway), and sorts the extracted
(value, row_id) stream once, in train_index, right before handing it to the target
trainer. Same correctness fix, one sort instead of two.

Testing

• test_json_float_btree_index_unsorted_input (rust/lance-index/src/scalar/json.rs):
  builds a real JSON/btree index through the plugin trainer/registry with the issue's
  repro values fed in raw storage order, and asserts range/equality results across 5
  cases. Also asserts JsonTrainingRequest::criteria().ordering == TrainingOrdering::None.
• test_json_index_non_exact_floats (python/python/tests/test_scalar_index.py): the
  issue's repro end to end via create_scalar_index + json_get_float filters.

cargo fmt --all, cargo clippy -p lance-index --tests -- -D warnings, and
uv run make lint are clean.

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[codecov]

Codecov Report

❌ Patch coverage is 93.54839% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-index/src/scalar/json.rs	93.54%	1 Missing and 1 partial ⚠️

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