Interpret \t in the indent setting as a tab

[shan3520]

Refs #2693

indent=\t in a config file was used literally, so wrapped imports were written as \tname, (a backslash followed by t), which is a SyntaxError in the output:

>>> print(isort.code(src, indent=r"\t", multi_line_output=3))
from a import (
\tbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb,
...

This makes \t in the indent value mean a tab, alongside the existing tab / number / literal-string handling. It also updates the --indent help text and docs/configuration/options.md to list the accepted values and to note that indent only applies to the hanging-indent multi_line_output modes, because GRID, VERTICAL and BACKSLASH_GRID align with the opening bracket. That explains the original report in #2693.

This is separate from #2704: on main, the quoted and capitalised tab spellings already work (details in my comment there). This PR only covers the \t case. I've left out the "reject an empty indent" idea from my issue comment, since it changes behaviour and is a maintainer call. I can add it here if you'd like.

Tests: parametrized Config(indent=...) cases plus one reading indent=\t from a real .isort.cfg. The unit suite passes locally (678 passed), and ruff and mypy are clean.

I used Claude to help investigate and write this change; I've reviewed and verified it.

[DanielNoord]

Are we sure the difference between multi line modes is not a bug? Can we use indent there? Or would that not make a lot of sense?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.41%. Comparing base (d93953f) to head (e8389c1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2708   +/-   ##
=======================================
  Coverage   99.41%   99.41%           
=======================================
  Files          41       41           
  Lines        3229     3230    +1     
  Branches      689      689           
=======================================
+ Hits         3210     3211    +1     
  Misses         12       12           
  Partials        7        7           

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[shan3520]

That's a good question. I would say it was done intentionally and not because of a bug. Grid, Vertical and Backslash Grid align the continuation lines with the column after from x import (: the white_space padding is calculated from the statement width in wrap.py, and that alignment is the main feature of those modes, as illustrated in multi_line_output_modes.md. If they used indent, the modes would turn into the existing hanging variants:

• Grid (0) → Vertical Grid (4)
• Vertical (1) → Vertical Hanging Indent (3)
• Backslash Grid (11) → Hanging Indent (2)

And tabs could not produce column alignment, since tab width depends on the editor.

So I would keep the current behaviour and use the note in the docs to let people discover it. If you want it to be more obvious, one idea is to show a warning when indent contains a tab and an aligned mode is selected. I'd be happy to implement it either here or in a follow-up.