Fix IndexError in traceback formatter

[kiri11]

Closes: #191

Describe your changes

Fix IndexError: list index out of range in format_frame when the source file on disk has fewer lines than the lineno recorded in the target process's frame. This can happen when the code object's line numbers don't correspond to the file's actual line count — for example, Jinja2 compiled templates use the template filename but with line numbers from the generated Python source, or when a file has been modified on disk since the process compiled it.

The fix adds a bounds check (1 <= lineno <= len(lines)) before indexing into the read lines. When lineno is out of range, the source line is skipped but the rest of the frame (arguments, locals) is still displayed.

Cases handled:

• Jinja2 mismatched line numbers (lineno 13, file has 7 lines) — skips source display
• Truncated/empty files (lineno > 0, file has 0 lines) — skips source display
• Shim frames (lineno = 0) — skips source display instead of silently showing lines[-1]
• Valid line numbers — unchanged behavior

Testing performed

Added test_traceback_formatter_with_source_lineno_out_of_range which constructs a frame pointing to line 13 of a file that is empty on disk, and verifies that format_frame does not raise and still renders the frame header, arguments, and locals.

Additional context

This also fixes a latent bug where lineno = 0 (used by shim frames) would silently index lines[-1], displaying the last line of the file as if it were the source — a misleading but non-crashing behavior.

Updated test_native_traceback_with_shim_frames to remove the bogus source lines that shim frames (lineno=0) were displaying. The old code indexed lines[0 - 1] which Python resolved to lines[-1], silently showing the last line of the file as "source" for the shim frame. The test was asserting this incorrect behavior.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.22%. Comparing base (b87b6b6) to head (67596b4).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #305      +/-   ##
==========================================
+ Coverage   79.19%   79.22%   +0.02%     
==========================================
  Files          51       51              
  Lines        5542     5550       +8     
  Branches      580      581       +1     
==========================================
+ Hits         4389     4397       +8     
  Misses       1153     1153              

Flag	Coverage Δ
cpp	79.22% <100.00%> (+0.02%)	⬆️
python	79.22% <100.00%> (+0.02%)	⬆️

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[sarahmonod]

Hi @kiri11 can you please run git commit --amend -s to add a sign off to your commit? Thanks!

[sarahmonod]

LGTM, thanks @kiri11!

[sarahmonod]

@kiri11 please rebase this branch on top of main instead of adding a merge commit. This keeps history clean and follows the project's conventions

[godlygeek]

LGTM, thanks @kiri11 !