fix: preserve the timezone in the schema when resolving output timezone

[nevi-me]

Which issue does this PR close?

• Closes to_timestamp_* discards the timezone of already-timezone-aware inputs #23841.

Rationale for this change

to_timestamp_millis (and the rest of the family) should not silently drop the timezone of an already-zoned input. Passing a Timestamp(_, Some("+05:00")) through the function should either:

1. preserve the input zone, or
2. if honoring datafusion.execution.time_zone, convert the timezone but never relabel to a naive type, which changes the wall-clock value.

What changes are included in this PR?

Restore the behaviour of preserving the input zone

Are these changes tested?

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

❌ Patch coverage is 95.83333% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.67%. Comparing base (8d2ffc9) to head (51c6d12).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
datafusion/functions/src/datetime/to_timestamp.rs	95.83%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23925      +/-   ##
==========================================
- Coverage   80.67%   80.67%   -0.01%     
==========================================
  Files        1095     1095              
  Lines      372460   372519      +59     
  Branches   372460   372519      +59     
==========================================
+ Hits       300495   300536      +41     
- Misses      54040    54054      +14     
- Partials    17925    17929       +4     

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