Fix #2561: Escape control characters in assertion error messages

[nohwnd]

Fix #2561

Format-String2 now escapes control characters as Unicode control pictures so they are visible in error messages. Characters like ESC (used in ANSI sequences) now display as their Unicode symbol instead of being invisible.

Copilot-generated fix.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

[nohwnd]

Rewrote at eb5917c per your comments — single StringBuilder pass, fast path for the common no-special-chars case:

• Module-load [char[]] $script:Format2ControlChars covering 0x00..0x1F.
• String.IndexOfAny for the cheap probe; if none, just "'$Value'" — no rebuild.
• Slow path: one StringBuilder, one foreach over GetEnumerator(), control chars get + 0x2400 (Unicode Control Pictures, same trick as Format-CDataString in NUnit3).

Micro-bench (50k calls, PS 7.6.2):

• fast path: ~1.4s
• slow path: ~6.8s

Side benefit: now escapes the entire 0x00..0x1F range instead of just 8 hand-listed chars — vertical tab, FS/GS/RS/US etc. also become visible.

Tests: 69/69 green on both PS 7.6 and PS 5.1.

[nohwnd]

Moved the hot loop to Pester.Formatter.EscapeControlChars (src/csharp/Pester/Formatter.cs) at d89c54d. Format-String2 is now a one-liner delegating to the C# static.

Correctness on 200,000-char input with 10k each of NUL/TAB/LF/ESC scattered through it — all 40k control chars converted to U+2400..U+241B, zero raw control chars left.

Bench (1k iterations, PS 7.6.2):

length	fast (no specials)	slow (1 ESC)	slow (all ESC)
100	81 ms	56 ms	98 ms
1,000	80 ms	107 ms	95 ms
10,000	422 ms	386 ms	382 ms
100,000	3,685 ms	3,754 ms	3,784 ms

For a typical short error-message string fast and slow paths are both ~0.08 ms per call. 69/69 tests green on PS 7.6 and PS 5.1.

[nohwnd]

Good call — pulled the <empty> check and quote-wrapping back out into Format-String2 at 6f6e181. The C# helper now only does what its name says (escape control chars; returns input unchanged when no escaping is needed). 69/69 still green on PS 7.6 and PS 5.1.

function Format-String2 ($Value) {
    if ($null -eq $Value -or $Value.Length -eq 0) {
        return '<empty>'
    }
    "'" + [Pester.Formatter]::EscapeControlChars($Value) + "'"
}