Don't panic on quoted text around a number placeholder

[keithadler]

Problem

Cell::formatted_value() panics on any numeric cell whose number format starts and ends with a quote but has a placeholder in between, such as "Qty "0" pcs" or "Total: "#,##0.00" EUR":

thread 'main' panicked at src/helper/number_format.rs:130:67:
called `Result::unwrap()` on an `Err` value: ParseFloatError { kind: Invalid }

to_formatted_string treats every format that starts and ends with " as a quoted number and calls format.trim_matches('"').parse::<f64>().unwrap(). For these formats the trimmed text is Qty "0" pcs, which is not a number. (Formats that are only quoted text, like " Excellent" in Apache POI's ConditionalFormattingSamples.xlsx, are already handled by literal_only_section on master; they panicked in 3.1.0.)

Change

Take that branch only when the quoted text really parses as a number. Everything else falls through to the number formatter, which already renders these the way Excel does: Qty 5 pcs, Total: 1,234.50 EUR.

Tests

test_to_formatted_string_quoted_text_around_a_placeholder in helper/number_format.rs; it panics without the change. cargo +nightly fmt --all --check, cargo clippy -- -D warnings and cargo test pass.

Found by differential testing with xlsx-lean and Apache POI's test files.

🤖 Generated with Claude Code

https://claude.ai/code/session_019hXzkiuCwk5QQRsyU9wT9h

[MathNya]

@keithadler
Thank you for the PR.
Since there were no issues, I’ll go ahead and merge it.