[BUG] Normalize decoder targets with encoder statistics

[mig-builds]

Reference Issues/PRs

Fixes #2360.

What does this implement/fix? Explain your changes.

EncoderDecoderTimeSeriesDataModule previously fitted
EncoderNormalizer on each encoder window and normalized target_past, but
returned the corresponding decoder target y without the sequence-local
normalization.

This PR:

• Adds ScalerAdapter.transform_sequence() to transform data using an
  already-fitted sequence-local normalizer.
• Fits normalization parameters exclusively on the encoder target window.
• Reuses those parameters to transform the decoder target.
• Preserves existing behavior for non-sequence normalizers and mixed
  multi-target configurations.
• Documents the normalization behavior in the v2 data documentation and API
  docstring.

The decoder target is never used when fitting the normalizer, preventing future
target leakage.

What should a reviewer concentrate their feedback on?

• Whether transform_sequence() has the appropriate behavior for single and
  multi-target normalizers.
• Whether reusing encoder-fitted statistics for the decoder target is consistent
  with the intended EncoderNormalizer semantics.
• Whether mixed MultiNormalizer configurations continue to preserve the
  existing behavior for non-sequence target columns.

Did you add any tests for the change?

Yes. Added test_encoder_normalizer_applies_to_decoder_target, which verifies
that:

• Decoder y uses the normalization parameters fitted on the encoder window.
• The normalized decoder target matches an independent transformation using the
  fitted state.
• The returned decoder target differs from the raw target.

The complete data-module test file passes:

• 43 tests passed
• Ruff checks passed
• git diff --check passed

Any other comments?

The documentation now explicitly states that sequence-local normalization is
fitted exclusively on encoder observations and then reused for both
target_past and decoder y.

PR checklist

• The PR title starts with either [ENH], [MNT], [DOC], or [BUG].
• Added/modified tests.
• Used pre-commit hooks to ensure that the code complies with repository
  hooks.

[codecov]

Codecov Report

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Additional details and impacted files

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