Implement SafeProcessHandle.WaitForExit* methods

[Copilot]

Implements the approved SafeProcessHandle.WaitForExit* API surface from #123380.

Description

Adds 5 new public methods to SafeProcessHandle: WaitForExit, TryWaitForExit, WaitForExitOrKillOnTimeout, WaitForExitAsync, and WaitForExitOrKillOnCancellationAsync.

Key changes

• Shared async logic in SafeProcessHandle.cs using RegisterWaitForSingleObject + TaskCompletionSource
• Windows: Uses ProcessWaitHandle + GetExitCodeProcess for synchronous wait and exit code retrieval
• Unix: Integrates with ProcessWaitState for reaping coordination; ProcessId property moved to platform-specific file and reads from _waitStateHolder._state._processId
• Native PAL: WaitPidExitedNoHang now returns terminating signal info via TryConvertSignalCodeToPosixSignal
• ProcessWaitState: Stores ProcessExitStatus? instead of separate _exitCode/_terminatingSignal fields; added Holder.IncrementRefCount() for ref-counted ownership sharing between Process and SafeProcessHandle
• Process.Unix.cs: GetProcessHandle() and StartCore use IncrementRefCount() pattern for proper wait state holder ownership
• Consistency fix: Both ProcessId and GetWaitState() now throw InvalidOperationException when _waitStateHolder is null (previously ProcessId threw PlatformNotSupportedException); GetWaitState() throws PlatformNotSupportedException for non-child processes on Unix
• Ref assembly: Updated with new API surface
• Tests: Comprehensive WaitForExit* tests including sync/async theories, signal reporting, timeout/kill behavior, non-child process handling, and Process.SafeHandle interop