Libs: (bug) Valid custom numeric format string is rejected with FormatException when a literal digit run after a leading letter exceeds 9 digits

Start in src/libraries/Common/src/System/Number.Formatting.Common.cs at Number.ParseFormatSpecifier and trace the digit-scanning path used by numeric ToString and TryFormat. Add regression coverage for long digit runs followed by a non-digit, confirming those custom formats work while a genuinely standard format with excessive precision still throws FormatException.

Number.ParseFormatSpecifier throws FormatException while scanning the precision digit run of a possible standard format, before it can see that the string is actually a custom format. So "A1111111111-0" is rejected while the otherwise identical "A111111111-0" (one digit fewer) formats fine.

Minimal Repro

using System;
using System.Globalization;

internal static class P
{
    private static void Show(string fmt)
    {
        try { Console.WriteLine($"\"{fmt}\" => \"{7.ToString(fmt, CultureInfo.InvariantCulture)}\""); }
        catch (Exception e) { Console.WriteLine($"\"{fmt}\" => {e.GetType().Name}"); }
    }

    private static void Main()
    {
        Show("A111111111-0");   // 9-digit literal run  -> custom format, works
        Show("A1111111111-0");  // 10-digit literal run -> same shape, throws
        Show("A123456780Z");    // works
        Show("A1234567890Z");   // throws
        Show("A1234567890");    // genuinely standard-shaped: FormatException is correct here
    }
}

Expected

A format string that is not <letter><optional precision> is a custom format string, so the trailing -0 / Z makes these custom and they must format:

"A111111111-0"  => "A111111111-7"
"A1111111111-0" => "A1111111111-7"
"A123456780Z"   => "A123456787Z"
"A1234567890Z"  => "A1234567897Z"
"A1234567890"   => FormatException     (correct: standard format, precision > 999,999,999)

Actual

"A111111111-0" => "A111111111-7"
"A1111111111-0" => FormatException
"A123456780Z" => "A123456787Z"
"A1234567890Z" => FormatException
"A1234567890" => FormatException

Notes

Root cause: in src/libraries/Common/src/System/Number.Formatting.Common.cs, the digit-scanning loop calls ThrowHelper.ThrowFormatException_BadFormatSpecifier() as soon as n >= 100_000_000, instead of remembering the overflow and falling through to the custom-format path. The throw is only correct when the scan actually ends at the end of the format (or at \0) — the condition the following if already tests.

Trigger: leading ASCII letter, a digit run reaching 100,000,000, followed by at least one non-digit. Affects all numeric types routed through this helper (int, long, float, double, Half, decimal, BigInteger, …) and both ToString and TryFormat. Also repros on .NET 10.0.7 and .NET 8.0.26; likely introduced by #72647 (.NET 7), which replaced the previous int-overflow-only check. No test covers a digit run followed by a non-digit.