Use SearchValues in PathString

[MihaZupan]

• Replace PathStringHelper.IsValidPathChar loops with SearchValues
• Add a vectorized fast-path for longer sections of valid chars
• Use existing Uri.IsHexEncoding helper instead of IsPercentEncodedChar
• Remove some dead code and eliminate some redundant length checks

Contributes to #46484

I'll post some benchmark numbers later today, but you can easily expect a 10x improvement for long already-valid values.

[MihaZupan]

Benchmark code

Method	Path	Mean	Error	StdDev	Ratio
Before	LongTestPath	283.907 ns	2.8201 ns	4.2210 ns	16.37
After	LongTestPath	17.331 ns	0.0495 ns	0.0693 ns	1.00
Before	LongT(...)rcent [24]	302.893 ns	0.7137 ns	1.0683 ns	11.90
After	LongT(...)rcent [24]	25.450 ns	0.0900 ns	0.1291 ns	1.00
Before	LongValidTestPath	244.094 ns	1.3209 ns	1.9771 ns	32.45
After	LongValidTestPath	7.523 ns	0.0330 ns	0.0483 ns	1.00
Before	TestPath	24.910 ns	0.2281 ns	0.3271 ns	1.83
After	TestPath	13.643 ns	0.0851 ns	0.1193 ns	1.00

With longer inputs, you can get over 40x, and that'll also get a bit better before .NET 8 ships if we sprinkle Avx512 around SearchValues by then.

[Tratcher]

components-e2e is being addressed: #49113

[halter73]

🚀

[mitchdenny]

Also related: #49114

[Tratcher]

/azp run

[azure-pipelines]

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