Optimize numeric-to-string coercion in StringArrayDecoder

[ndemir]

Which issue does this PR close?

I see an issue opened here: #7273
I realized that can be optimized and I worked on that (not in the way that was suggested in the issue).

Closes #7273

Rationale for this change

This PR introduces performance improvements for string coercion.

Performance Metrics:

Author of the #7273 recommends to use cargo bench --bench json_reader, we will go with that.

Step#1: Save Baseline Metrics

> git checkout main
> cargo bench --bench json_reader -- --save-baseline before        

Step#2: Compare the changes with Baseline Metrics

> git checkout ndemir/string-coerce-optimization
> cargo bench --bench json_reader --  --baseline before

small_bench_primitive   time:   [8.5790 µs 8.5890 µs 8.6029 µs]
                        change: [-3.0652% -2.4764% -1.9676%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 4 outliers among 100 measurements (4.00%)
  2 (2.00%) high mild
  2 (2.00%) high severe

large_bench_primitive   time:   [3.2421 ms 3.2951 ms 3.3615 ms]
                        change: [+0.9382% +2.6439% +4.5593%] (p = 0.00 < 0.05)
                        Change within noise threshold.
Found 9 outliers among 100 measurements (9.00%)
  1 (1.00%) high mild
  8 (8.00%) high severe

small_bench_list        time:   [17.444 µs 17.655 µs 18.076 µs]
                        change: [-6.4619% -4.0195% -2.1687%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 5 outliers among 100 measurements (5.00%)
  1 (1.00%) low severe
  1 (1.00%) high mild
  3 (3.00%) high severe

What changes are included in this PR?

• Shared buffer for numeric -> string: Added a number_buffer to reuse for numeric->string conversions.
• Performance: This reduces overhead by writing numeric values to a single buffer and then converting to &str.
• Safety comment: I added a comment and explain why from_utf8_unchecked is valid.

Are there any user-facing changes?

No.

[ndemir]

I see that the performance is consistently BETTER even though it is not hitting the refactored code.
Will investigate.

[alamb]

I think you can avoid unsafe here by using

    number_buffer: String,

This is similar to how @zhuqi-lucas did it here:

• feat: Support Utf8View in JSON reader #7263

        // Temporary buffer to avoid per-iteration allocation for numeric types
        let mut tmp_buf = String::new();
...