### Context

The NodeMode enum was in Shared namespace, duplicated logic existed in
DebugUtils, and parsing used string.Format placeholders instead of
direct construction.

### Changes Made

- **Moved NodeMode to Framework**: Relocated from
`Microsoft.Build.Shared` to `Microsoft.Build.Framework` for better
accessibility
- **Added NodeModeHelper.TryParse()**: Safe parsing supporting both
integers (1, 2, 3, 8) and enum names (case-insensitive) with
`Enum.IsDefined` validation
- **Refactored XMake.ParseNodeMode()**: Now uses TryParse pattern, made
internal for testability, preserves original error messages
- **Unified DebugUtils with NodeMode**: Removed the private
`ProcessMode` enum from DebugUtils and replaced it with `NodeMode?` from
Framework. The nullable type allows `null` to represent the central/main
MSBuild process (not running as a node), while actual node modes use the
shared enum. Updated parsing to use `NodeModeHelper.TryParse()` for
consistency across the codebase.
- **Cleaned up command line construction**: Replaced string.Format
placeholders in NodeProviderOutOfProcTaskHost with direct string
interpolation matching NodeProviderOutOfProc pattern

### Testing

Added 6 unit tests covering:
- Valid integers: 1, 2, 3, 8
- Valid enum names: case-sensitive and case-insensitive
- Error cases: undefined integers, negative numbers, invalid strings,
empty strings

### Notes

- The `Enum.IsDefined` check after `Enum.TryParse` is required because
TryParse succeeds for string representations of undefined integer values
(e.g., "5" parses successfully even though only 1, 2, 3, and 8 are
defined).
- DebugUtils now uses `NodeMode?` (nullable) where `null` represents the
central/main process, eliminating the need for a separate `ProcessMode`
enum and ensuring a single source of truth for node mode values.

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: baronfel <573979+baronfel@users.noreply.github.com>