fix: Unnest must not keep its input's uniqueness

[Dandandan]

Which issue does this PR close?

• Closes #.

Rationale for this change

Unnest::try_new copies its input's functional dependencies unchanged:

// We can use the existing functional dependencies:
let deps = input_schema.functional_dependencies().clone();

That is not true for a list unnest, which turns one input row into several. A
determinant that occurred once in the input can occur many times in the output,
so it is no longer a unique key.

Optimizer rules that read those dependencies then produce wrong results:

CREATE TABLE t_list (k INT, vals INT[], PRIMARY KEY (k))
  AS VALUES (1, [10, 20, 30]), (2, [40]);
CREATE TABLE t_join (k INT) AS VALUES (1), (2);

SELECT u.k, u.v FROM (SELECT k, unnest(vals) AS v FROM t_list) u
JOIN t_join j ON u.k = j.k;

returns 2 rows. The correct answer is 4, and removing the PRIMARY KEY gives
4. eliminate_join sees u as unique on k, rewrites the inner join into a
semi join, and the repeated rows are dropped.

What changes are included in this PR?

The dependency still holds in the weaker sense: every row produced from one
input row carries the same determinant, so it determines the same columns. It
is downgraded to Dependency::Multi rather than dropped, which keeps it useful
for other purposes and stops it being read as a uniqueness guarantee.

Unnesting a struct produces one row per input row, so those dependencies are
left as they are.

Are these changes tested?

Yes. functional_dependencies.slt gains the query above, which returns 4 rows,
and a plan assertion that a struct unnest still keeps its input's dependencies.
The full sqllogictest suite (504 files) passes.

Are there any user-facing changes?

Queries that unnest a list from a table with a declared key and then join or
aggregate on that key now return correct results.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.52%. Comparing base (61bf6b9) to head (f9ad1c3).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #24787   +/-   ##
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  Coverage   81.52%   81.52%           
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  Files        1123     1123           
  Lines      405970   406044   +74     
  Branches   405970   406044   +74     
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+ Hits       330978   331040   +62     
- Misses      55627    55638   +11     
- Partials    19365    19366    +1     

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