fix(spark): format_string %s of a DECIMAL and %g like Java's Formatter

[ozenyunus35]

Which issue does this PR close?

• Closes [Bug] format_string prints the unscaled value for %s of a DECIMAL and strips trailing zeros for %g #26337.

Rationale for this change

Spark's format_string uses Java's Formatter, and datafusion-spark differed from it in two ways: %s of a DECIMAL printed the unscaled integer (123456 for 123.456), and %g / %G dropped trailing zeros like C (1234.5), while Java always prints the requested number of significant digits (1234.50).

What changes are included in this PR?

• %s / %S of a Decimal128 or Decimal256 prints the value with its scale in plain notation (123.456, 0.50), through a small decimal_to_plain_string helper.
• %g / %G no longer strip trailing zeros, for float and decimal arguments. The strip_trailing_0s flag and trim_trailing_0s were only used for this, so they are removed.
• While adding cases, %g of 0.0 panicked with "attempt to subtract with overflow", because the exponent came from log10(0). A decimal_exponent helper returns 0 for zero, so %g gives 0.00000 and %e gives 0.000000e+00, as in Java.

I checked every new expected value by running String.format with the same pattern on JDK 21 (BigDecimal for the decimal %s cases).

One thing I left as is: Java's BigDecimal.toString switches to E notation for very small values (1E-7 for 0.0000001); this PR prints those in plain notation. I couldn't confirm what Spark's own Decimal.toString gives there without a Spark install, so I didn't guess.

What is the testing strategy for this PR?

• format_string.slt: the expectations the issue points at are updated (%s / %S of Decimal128 and Decimal256, %g / %G of Float64, Float32, Decimal128 and Decimal256), and new cases cover %g on small, large, zero, negative and rounded values, %.3g, %.0g, %10.2g, %e of zero, and %s / %S / %10s of decimals. On main the file fails (and the %g of 0.0 case panics).
• cargo test -p datafusion-spark --lib format_string and cargo clippy -p datafusion-spark --all-targets -- -D warnings pass.

Are there any user-facing changes?

Yes: format_string('%s', <decimal>) prints the decimal with its scale, %g / %G keep trailing zeros, and %g of zero no longer panics.

I used an AI assistant to help draft parts of the code and tests; I reviewed the changes and ran the tests myself.