V4A patches with only + lines (no context or - lines) were silently
dropped because search_lines was empty and the 'if search_lines:' block
was the only code path. Addition-only hunks are common when the model
generates patches for new functions or blocks.
Adds an else branch that inserts at the context_hint position when
available, or appends at end of file.
Includes 2 regression tests for addition-only hunks with and without
context hints.
Salvaged from PR #3092 by thakoreh.
Co-authored-by: Hiren <hiren.thakore58@gmail.com>