Add @overload type signatures for series2seq and seq2series

[authierj]

Description

Checklist before merging this PR:

• Mentioned all issues that this PR fixes or addresses.
• Summarized the updates of this PR under Summary.
• Added an entry under Unreleased in the Changelog.

Summary

Improves static type inference for the two TimeSeries sequence-conversion helpers in
darts/utils/ts_utils.py:

• Added @overload signatures to series2seq() so the return type is narrowed based on seq_type_out (SeriesType.SEQ → Sequence[TimeSeries], SeriesType.SEQ_SEQ →Sequence[Sequence[TimeSeries]]) and on whether ts is None.
• Added @overload signatures to seq2series(), and corrected its implementation return annotation from TimeSeries | None to TimeSeriesLike | None (it returns the sequence unchanged when it holds more than one element).
• Expanded the Returns docstring sections of both functions to document the None case and the fact that down-conversions only unwrap single-element sequences.

No runtime behavior changes — annotations and docstrings only.

Other Information

The overloads are currently grouped by confidence (GOOD ONES / OKAY / TO BE IMPROVED)
via inline comments. The broad catch-all overloads are needed until callers get their own
overloads, and can be narrowed in follow-up work.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.96%. Comparing base (080b534) to head (2b01536).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3178      +/-   ##
==========================================
- Coverage   97.03%   96.96%   -0.07%     
==========================================
  Files         165      165              
  Lines       17951    17952       +1     
==========================================
- Hits        17418    17408      -10     
- Misses        533      544      +11     

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