Make the session-timezone comparison rule public, and deprecate the two helpers it orphans

[adriangb]

Which issue does this PR close?

Nothing on its own — this targets your fix/13212-timezone-inference branch, i.e. it is a
follow-up to apache#25094. Merging it updates that PR.

Rationale for this change

0390171 reaches IN and CASE expr WHEN by inlining the folds that used to live in
get_coerce_type_for_list and get_coerce_type_for_case_when. That leaves two things behind:

1. The rule itself — "IN coerces the way = does, session timezone included" — exists only
   as a private function inside datafusion-optimizer.
2. Both pub helpers still answer the question "how does IN coerce?", with an answer
   DataFusion itself no longer uses.

Those helpers aren't dead code. A GitHub search turns up two non-fork callers of
get_coerce_type_for_list: Canner/WrenAI, which vendors a copy of this very rewriter,
and waynexia/jsonfusion, which uses it for its own IN-list inference.
get_coerce_type_for_case_when has no external callers I can find.

An earlier version of this PR gave both helpers a session_time_zone parameter instead. That
is a breaking change — api-health.md lists "adding new required parameters" first
among breaking changes, and lists "an internal refactor to make DataFusion more consistent"
among the weak reasons to make one. It also wouldn't have helped those two callers: they'd
pass None and land exactly where they started. Deprecation is non-breaking, gives them a
warning plus a documented replacement, and stays reversible.

What changes are included in this PR?

Two commits:

1. Give the session-timezone comparison rule one public home. Moves
   comparison_coercion_with_session_timezone out of the analyzer and into expr-common,
   next to comparison_coercion itself, as pub. The analyzer calls it in the same three
   places; it just no longer owns the rule. Adds a unit test covering the None session
   timezone and the Null-versus-dictionary case, where routing through = differs from
   comparison_coercion.

2. Deprecate get_coerce_type_for_list and get_coerce_type_for_case_when
   (since = "56.0.0"), pointing at that function, with a worked example of the replacement
   fold in the rustdoc and an entry in the 56.0.0 upgrade guide.

The analyzer's coercion behavior is unchanged by both commits — this is API shape only.

Are there any user-facing changes?

No breaking changes. One new public function, two deprecations, and an upgrade-guide entry.
Per the API health policy the deprecated functions stay for six major versions, so nothing
has to move before 62.0.0.

Verified locally: cargo fmt --all -- --check; cargo clippy --all-targets -D warnings and
the unit tests for datafusion-expr-common, datafusion-expr and datafusion-optimizer;
cargo test -p datafusion-expr --doc; and 76 sqllogictest files including
datetime/timestamps.slt.

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