Fix Complex number custom formatting for negative imaginary parts

[Copilot]

The ComplexFormatter examples in the documentation were producing incorrect output when formatting complex numbers with negative imaginary parts. Instead of displaying 12 - 15i, they would show 12 + -15i.

Problem

The custom formatter always used a hardcoded " + " operator between the real and imaginary parts:

// C# - Before
return c1.Real.ToString(fmtString) + " + " + c1.Imaginary.ToString(fmtString) + "i";

// VB.NET - Before  
Return c1.Real.ToString(fmtString) + " + " + c1.Imaginary.ToString(fmtString) + "i"

When c1.Imaginary was negative, ToString() would include the negative sign, resulting in output like "12 + -15i" instead of the expected "12 - 15i".

Solution

Updated both C# and VB.NET versions to:

1. Calculate the appropriate sign based on the imaginary part's value
2. Use Math.Abs() to get the absolute value of the imaginary part
3. Display the calculated sign separately

// C# - After
char sign = c1.Imaginary < 0 ? '-' : '+';
return c1.Real.ToString(fmtString) + " " + sign + " " + Math.Abs(c1.Imaginary).ToString(fmtString) + "i";

' VB.NET - After
Dim sign As Char = If(c1.Imaginary < 0.0, "-"c, "+"c)
Return c1.Real.ToString(fmtString) + " " + sign + " " + Math.Abs(c1.Imaginary).ToString(fmtString) + "i"

Test Results

• Positive imaginary: 12 + 15i ✅ (unchanged)
• Negative imaginary: 12 - 15i ✅ (fixed from 12 + -15i)
• Zero imaginary: 12 + 0i ✅ (unchanged)

The fix applies to both 'I' and 'J' formatting cases and maintains backward compatibility for positive and zero imaginary parts.

Fixes #46422.

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[BillWagner]

This LGTM @gewarren

Let's