Raise error if two-echo data are provided

[tsalo]

Replaces #3569. Relates to #3364. This acts as a short-term patch until we figure out how best to process two-echo fMRI data.

Changes proposed in this pull request

• If two-echo fMRI data are provided and --echo-idx is not, raise a RuntimeError.

[effigies]

LGTM

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.88%. Comparing base (e3c6dfd) to head (1ea45f4).
⚠️ Report is 2 commits behind head on maint/25.2.x.

Files with missing lines	Patch %	Lines
fmriprep/workflows/base.py	0.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           maint/25.2.x    #3570      +/-   ##
================================================
- Coverage         72.92%   72.88%   -0.05%     
================================================
  Files                60       60              
  Lines              4817     4820       +3     
  Branches            625      626       +1     
================================================
  Hits               3513     3513              
- Misses             1162     1164       +2     
- Partials            142      143       +1     

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