Fix unindented comments being corrupted in indented blocks

When an unindented comment appears before an indented import block,
isort blindly strips `len(indent)` characters from the start of every
line. This corrupts comments that don't start with the expected indent.

For example:
```python
if True:
# this will get cut off
    import os
```
became:
```python
if True:
is will get cut off
    import os
```

The fix checks whether each line actually starts with the indent
before stripping it.

Fixes #1899

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Friday

isort/core.py

@@ -420,7 +420,8 @@ def process(
 
                     if indent:
                         import_section = "".join(
-                            line[len(indent) :] for line in import_section.splitlines(keepends=True)
+                            line[len(indent) :] if line.startswith(indent) else line
+                            for line in import_section.splitlines(keepends=True)
                         )
 
                     parsed_content = parse.file_contents(import_section, config=config)

tests/unit/test_ticketed_features.py

@@ -1073,3 +1073,17 @@ def use_libc_math():
 """,
         show_diff=True,
     )
+
+
+def test_unindented_comment_in_indented_block_issue_1899():
+    """Test that unindented comments before indented imports are not corrupted.
+
+    See: https://github.com/PyCQA/isort/issues/1899
+    """
+    test_input = """import sys
+
+if True:
+# this will get cut off
+    import os
+"""
+    assert isort.code(test_input) == test_input