bug: consecutive partial numbers (.1 followed by .2) wrongly merged into '.1 .2'

Bug

In _merge_partial_numbering_lines() (_pdf_converter.py), when two partial MasterFormat-style numbers appear on consecutive lines, the function merges the first number with the second number instead of merging it with the actual text below.

Reproduction

`python
from markitdown.converters._pdf_converter import _merge_partial_numbering_lines

text = '.1\n.2\nContractor shall furnish all materials.\n.3\nWork shall comply with local codes.'
print(_merge_partial_numbering_lines(text))
`

Actual output:
.1 .2 Contractor shall furnish all materials. .3 Work shall comply with local codes.

Expected output:
.1 .2 Contractor shall furnish all materials. .3 Work shall comply with local codes.

Root cause

Line 47 in _pdf_converter.py merges the current partial number with the next non-empty line unconditionally — it never checks if that next line is itself a partial number.

Fix

Add one guard before merging:
python if j < len(lines) and not PARTIAL_NUMBERING_PATTERN.match(lines[j].strip()):

PR: #2113

Start in _pdf_converter.py at _merge_partial_numbering_lines() and review the handling of the next non-empty line against PARTIAL_NUMBERING_PATTERN. Run the reproduction from the issue and verify that consecutive partial numbers remain separate while each number still merges with following text. The issue references PR #2113, so check that work before starting.