Fix: preserve bare # inline comments on imports

[Copilot]

import a # was being silently rewritten to import a — bare # comments were dropped entirely during sort/reformat.

Root cause: comments.parse() returned "" for both "no # present" and "empty comment after #". All callers used truthy checks (if comment:), so bare # was indistinguishable from no comment and got discarded.

Changes:

• isort/comments.py — parse() now returns None (no #) vs "" (bare #) vs "text" (comment with content). add_to_line() handles the empty-string case by emitting # without a trailing space.
• isort/parse.py — All if comment / if associated_comment / if extra_line.comment checks updated to is not None.
• isort/output.py — Nested comment rendering and combine_straight_imports path fixed to correctly emit bare #.
• tests/unit/test_ticketed_features.py — Regression tests for straight imports, from-imports, and idempotency.

# Before
isort.code("import a  #\n")          # → "import a\n"   ❌
isort.code("from foo import bar  #\n") # → "from foo import bar\n"  ❌

# After
isort.code("import a  #\n")          # → "import a  #\n"   ✅
isort.code("from foo import bar  #\n") # → "from foo import bar  #\n"  ✅

Original prompt

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This section details on the original issue you should resolve

<issue_title>isort strips empty comments, e.g. import a # -> import a</issue_title>
<issue_description>This is merely weird in isolation, but more practically makes it difficult for downstream tools like shed to performantly compose isort with other refactoring passes.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes isort strips empty comments, e.g. import a # -> import a #1913

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[codecov]

Codecov Report

❌ Patch coverage is 90.47619% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.18%. Comparing base (aade4b3) to head (2646b88).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2488      +/-   ##
==========================================
- Coverage   99.25%   99.18%   -0.07%     
==========================================
  Files          40       40              
  Lines        3079     3084       +5     
  Branches      671      672       +1     
==========================================
+ Hits         3056     3059       +3     
- Misses         13       14       +1     
- Partials       10       11       +1     

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