i.spectral: fix text output to write band values

[Valyrian-Code]

Description

write2textf iterated with for row in enumerate(what), which binds row to
the (index, row) tuple. As a result the -t text output wrote a Python tuple
repr — including a redundant index and the easting/northing/label columns —
instead of the band values:

1, (0, [5.0, 5.0, 0, 10.0, 20.0, 30.0])

This unpacks the index and writes row[3:] (the band DN values), matching what
draw_gnuplot and draw_linegraph already do, so the output becomes:

1, [10.0, 20.0, 30.0]

Motivation and context

The text export silently produced unusable data (a Python repr rather than the
spectral values shown in the plots). The exit code was still 0, so nothing
flagged it.

How has this been tested?

Reproduced locally before and after the change. Added a regression test
(scripts/i.spectral/tests/i_spectral_test.py) that runs i.spectral -t on
three constant-value rasters and asserts the output line is
1, [10.0, 20.0, 30.0]; it fails on the previous code and passes on the fix.
ruff format and ruff check are clean.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Tests

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Fixes the i.spectral -t text output regression where tuple representations were written instead of band values, and adds a regression test to prevent recurrence.

Changes:

• Update write2textf() to write only band values (and correct pick numbering) for text output.
• Add a pytest regression test validating the expected text output format for a simple 3-band case.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
scripts/i.spectral/i.spectral.py	Fixes text output writing logic to avoid (index, row) tuple output and emit band values instead.
scripts/i.spectral/tests/i_spectral_test.py	Adds regression test covering the corrected -t text output formatting.

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