BUG: find_filename misses filenames when fenced block markers are indented

[Jingyi1003]

Summary

find_filename can miss a valid filename when the surrounding fenced block markers have leading whitespace.

Aider already supports recovering filenames from fenced blocks, and the find_filename docstring includes an indented fence example. In the current implementation, the loop stops early because it checks the raw line with line.startswith("```") instead of checking the stripped line.

Steps to reproduce

Check out Aider main at commit 5dc9490bb35f9729ef2c95d00a19ccd30c26339c.

Add the following test inside TestUtils in tests/basic/test_editblock.py:

def test_find_filename_with_indented_fences(self):
    fence = ("```", "```")
    valid_fnames = ["dir/file3.py"]

    lines = ["  ```python", "file3.py", "  ```"]

    self.assertEqual(
        eb.find_filename(lines, fence, valid_fnames),
        "dir/file3.py",
    )

Run:

python -m pytest tests/basic/test_editblock.py -q

Expected behavior

find_filename should treat indented fenced markers the same way as non-indented fenced markers when looking back for a filename, and return:

dir/file3.py

Actual behavior

The added regression test fails because find_filename returns None:

FAILED tests/basic/test_editblock.py::TestUtils::test_find_filename_with_indented_fences
AssertionError: None != 'dir/file3.py'

1 failed, 25 passed in 1.32s

Impact

When an LLM returns a SEARCH/REPLACE block with slightly indented fenced markers, Aider may fail to recover the filename even though the filename is present and matches a known repo file. This can make an otherwise valid edit block fail with a missing filename error.

Root cause

Inside find_filename, each candidate line is first normalized by strip_filename(line, fence), which strips whitespace before checking for fences.

But the continuation check uses the raw line:

if not line.startswith(fence[0]) and not line.startswith(triple_backticks):
    break

For an indented fence like " ```", this condition becomes true and the loop breaks before it reaches the filename line.

Possible fix direction

Use a stripped line for the fence-continuation check:

stripped = line.strip()
if not stripped.startswith(fence[0]) and not stripped.startswith(triple_backticks):
    break

A regression test should cover both plain fenced blocks and indented fenced blocks.

Version and model info

Aider: 0.86.3.dev, main@5dc9490bb35f9729ef2c95d00a19ccd30c26339c
Python: focused unit-level reproduction
Operating system: macOS
Installation: source checkout
Edit format: search/replace
Model: not applicable; reproduced by directly invoking deterministic filename parsing code