Add some tests for shared mutex corner cases

[jkoritzinsky]

Will help with validating the behaviors in #117635

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[Copilot]

Pull Request Overview

This PR adds test coverage for shared mutex corner cases on Unix platforms to validate behaviors related to named mutex implementation. The changes include adding a new native interop function to get system page size and implementing several test cases that validate error handling for invalid shared memory files.

Key changes:

• Added native SystemNative_GetPageSize() function to retrieve system page size
• Created managed interop wrapper for accessing page size from C#
• Added comprehensive test cases for named mutex error scenarios on Unix

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/native/libs/System.Native/pal_process.h	Added declaration for SystemNative_GetPageSize function
src/native/libs/System.Native/pal_process.c	Implemented SystemNative_GetPageSize using getpagesize()
src/native/libs/System.Native/entrypoints.c	Registered the new function in the DLL import table
src/libraries/System.Threading/tests/System.Threading.Tests.csproj	Added reference to the new interop file for page size functionality
src/libraries/Common/src/Interop/Unix/Interop.GetPageSize.cs	Created managed wrapper with lazy initialization for page size access
src/libraries/System.Threading/tests/MutexTests.cs	Added test cases for invalid mutex names and corrupted shared memory scenarios

[jkotas]

LGTM once the build breaks are fixed

[jkoritzinsky]

/ba-g DNS on build agent failure