…X failing with "UnionTermIterator does not support scoring" (#69699)

### What problem does this PR solve?

Issue Number: None

Related PR: #60331

Problem Summary:
SELECT score() FROM t WHERE c1 MATCH_PHRASE_PREFIX 'apple r' ORDER BY
score()
DESC LIMIT 10 fails with "[E3] UnionTermIterator does not support
scoring",
while the same filter without score() works.

The last term of a phrase prefix is a UnionTermIterator over the terms
the
prefix expands to. PhraseQuery sorts its iterators by document frequency
and
scores a row with the norm of the first one. When the terms the prefix
expands
to are rarer than another term of the phrase in a segment, the union
comes
first and UnionTermIterator::norm() threw. A union's document frequency
is the
sum of its terms', so whether a query fails depends on the data of each
segment. On a table holding "apple red", "apple green" and "banana
yellow",
`apple r` (red in 1 row, apple in 2) and `apple g` fail; on a table
where
`r*` matches more rows than apple the same query works. This regressed
when
the phrase-level BM25 scoring of #60331 (#61243 in 4.0) started reading
the
norm of the first iterator.

A norm belongs to the document, not to a term, so the sub-iterator
positioned
on the row, the top of the union's queue, reports it. The union now
returns
that norm.

Result on a branch-4.1 cluster built with this change: the reported
query
returns 0.4700036 and `apple g` returns a score; on a table where
branch-4.1 (4.1.5) threw for `apple gr`, the query now returns the row
that
the same filter without score() returns.

This is not a cherry-pick: master replaced the phrase query engine, and
the
same query there scores the prefix on the tablet idf of the terms it
expands to
(#69698).

### Release note

Fix score() over MATCH_PHRASE_PREFIX failing with "UnionTermIterator
does not
support scoring" when the prefix matches fewer rows than the other
terms.

### Check List (For Author)

- Test: Regression test, Unit Test
- Regression test test_match_phrase_prefix_score on a local branch-4.1
      cluster.
    - BE unit tests UnionTermIteratorTest.* and PhraseQueryTest.* pass,
      including the new NormIsReadFromTheSubIteratorOnTheDocument.
- Behavior changed: Yes (the query no longer fails)
- Does this need documentation: No

### Notes for reviewers

The score this branch returns is still the phrase scored on the idf of
its
plain terms (`apple r` returns 0.4700036); it does not count the terms
the
prefix expands to. That is left as it was, because it needs the
statistics and
engine changes of master.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Sonnet 5.5 <[email protected]>