Fix CompositeFormat brace escaping ignored in string.Format fast-path

[Copilot]

Fixes #127794

string.Format with CompositeFormat returned the raw format string instead of unescaping {{→{ and }}→} when no format holes were present.

Root Cause

Two fast-paths in String.Manipulation.cs short-circuit to format.Format (the original unparsed string) without checking whether brace escaping occurred:

1. args.Length == 0 branch returns format.Format unconditionally
2. _formattedCount == 0 guard in the private Format<TArg0,TArg1,TArg2> returns format.Format unconditionally

StringBuilder.AppendFormat and MemoryExtensions.TryWrite lack these fast-paths and work correctly by always iterating parsed segments.

Fix

Both conditions additionally check format._literalLength == format.Format.Length. When escaped braces are present, the unescaped literal length is shorter than the raw format string, so the condition is false and execution falls through to the correct segment-iteration path.

// Before:
0 => format.Format,

// After:
0 => format._literalLength == format.Format.Length
    ? format.Format
    : Format(provider, format, (object?)null, 0, 0, args),

// Before:
if (format._formattedCount == 0) { return format.Format; }

// After:
if (format._formattedCount == 0 && format._literalLength == format.Format.Length)
{ return format.Format; }

Test Coverage

Three new entries in Format_Valid_TestData (reused by StringFormat_Valid, StringBuilderAppendFormat_Valid, and MemoryExtensionsTryWrite_Valid):

Format	Expected
"{{"	"{"
"}}"	"}"
"{{text}}"	"{text}"

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[tarekgh]

/ba-g the failing iOS tests are not related.