[RuntimeAsync] Use truncating sprintf to make resumption stub names

[VSadov]

sprintf_s pops a crt assert dialog in checked/debug if the buffer is too short.
_snprintf_s with _TRUNCATE does not have such problem.

We may see some pretty long names here.

An example that was triggering the assert in #119432 was:
System.Collections.Generic.IAsyncEnumerator<System.ValueTuple<Microsoft.CodeAnalysis.Project,System.Collections.Immutable.ImmutableArray<System.ValueTuple<Microsoft.CodeAnalysis.ISymbol,Microsoft.CodeAnalysis.FindSymbols.SymbolGroup>>>>.MoveNextAsync

[Copilot]

Pull Request Overview

This PR fixes a CRT assert dialog issue in debug/checked builds when creating async resumption stub names. The change replaces sprintf_s with _snprintf_s using the _TRUNCATE option to handle cases where method names are too long for the fixed-size buffer.

Key changes:

• Switches from sprintf_s to _snprintf_s with _TRUNCATE flag to prevent CRT assertion dialogs
• Maintains the same buffer size and error checking logic

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[VSadov]

/ba-g the failures are all #120577