Fixes #12610 

### Context

Follow-up to #12577. The taskhost node identification used bit-packing
to combine `scheduledNodeId` (8 bits) and `HandshakeOptions` (9 bits)
into a single int, limiting multithreaded builds to 256 nodes. This
approach was fragile and hard to maintain.

### Changes Made

- **New `TaskHostNodeKey` record struct** in
`CommunicationsUtilities.cs` combining `HandshakeOptions` and `NodeId`
as a clean, type-safe key
- **Updated `NodeProviderOutOfProcTaskHost`** with optimized dictionary
structure:
- `_nodeContexts` keyed by `TaskHostNodeKey` for checking if a node with
given (HandshakeOptions, scheduledNodeId) exists (acquisition logic)
  - `_nodeIdToNodeKey` for O(1) reverse lookup during node termination
- `_nodeIdToPacketFactory` and `_nodeIdToPacketHandler` keyed by int for
O(1) packet routing performance
  - `_activeNodes` tracks node IDs (int) for shutdown handling
- Added atomic `_nextNodeId` counter for unique communication node ID
generation
- **Simplified `TaskHostTask`** - removed `GenerateTaskHostNodeId`
method and associated bit-packing constants
- **Improved error handling** - Added `VerifyThrow` assertions in
`DisconnectFromHost` and `CreateNode` to verify context existence and
prevent duplicate nodes

```csharp
// Before: bit-packed int with 256-node limit
_taskHostNodeId = GenerateTaskHostNodeId(_scheduledNodeId, _requiredContext);
// (scheduledNodeId << 9) | ((int)handshakeOptions & 0x1FF)

// After: clean record struct, no limit
_taskHostNodeKey = new TaskHostNodeKey(_requiredContext, _scheduledNodeId);
```

This gives the best of both worlds:
- **Maintainability**: Type-safe Keys for managing node lifecycle and
acquisition
- **Performance**: Direct Integer lookups for the high-frequency packet
routing hot path

### Testing

- Added 7 unit tests for `TaskHostNodeKey` (equality, hash codes,
dictionary keys, large node IDs)
- All 86 existing TaskHost tests pass

### Notes

- Removes the `NODE_ID_MAX_VALUE_FOR_MULTITHREADED` (255) constraint
entirely
- `SendData(int, INodePacket)` throws `NotImplementedException` as it's
required by `INodeProvider` interface but not used for task hosts
(similar to `TaskHostNodeManager`)

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>[multithreaded] Refactor Node IDs for
taskhost</issue_title>
> <issue_description>followup to #12577
> Refactor
src/Build/BackEnd/Components/Communications/NodeProviderOutOfProcTaskHost.cs
, HandshakeOptions to use record struct as a key to nodes to increase
maintanability and remove limitation to 256 nodes.</issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>

- Fixes #12610

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---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: JanProvaznik <[email protected]>
Co-authored-by: Jan Provazník <[email protected]>
Co-authored-by: Copilot <[email protected]>