[clr-interp] Implement missing support for startup path

[kotlarmilos]

Description

This PR includes several improvements to support interpreting methods on startup path:

• Refactor string allocation to use a native-sized integer instead of a fixed-size integer type
• Add support for CPBLK operation in the interpreter
• Special-case constructors for RuntimeMethodHandle, RuntimeFieldHandle, and similar types to support value-type semantics

Contributes to #117556

[Copilot]

Pull Request Overview

This PR implements missing support for interpreting methods on the startup path by adding CPBLK instruction support, refactoring string allocation to use native-sized integers, and handling special constructor cases for runtime handle types.

Key changes:

• Adds CPBLK (copy block) instruction implementation to the interpreter
• Changes string allocation methods from int to nint for length parameters
• Special-cases constructors for RuntimeMethodHandle, RuntimeFieldHandle, and RuntimeArgumentHandle to use value-type semantics

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/coreclr/vm/interpexec.cpp	Implements CPBLK instruction execution using memcpy
src/coreclr/interpreter/intops.def	Defines CPBLK instruction metadata
src/coreclr/interpreter/compiler.cpp	Adds CPBLK compilation support and special constructor handling
src/coreclr/System.Private.CoreLib/src/System/String.CoreCLR.cs	Changes string allocation length parameter from int to nint

[janvorli]

LGTM, thank you!

[janvorli]

/ba-g The System.Net.WebSockets.Client.Tests.CancelTest_SharedHandler_Loopback.ConnectAsync_Cancel_ThrowsCancellationException test failure is unrelated and happens because of a timeout.