Chart <a:t> elements (title, axis labels, series names) were collected and
sent to the LLM, but the apply loop never invoked their setters. Reason:
the apply loop iterated only text_elements[:sheet_name_offset], which
excluded the chart-text positions (sheet names were inserted in the
middle, pushing chart texts past sheet_name_offset). The LLM correctly
returned French for "Revenue by Product" / "Quantity Trend" / "Order #"
but the result was thrown away.
Fix: include the chart-text slice [sheet_name_offset + N .. total_texts]
in the apply loop. Setters for sheet names are still None, so they are
naturally skipped.
Add test_excel_chart_text_applied.py (2 end-to-end tests) using a fixed
provider that pre-translates every known chart text; the test asserts
all chart <a:t> values in the output .xlsx come from the FR table, not
the original English source.
