Phi Silica ignoring stop token

Nobody has claimed this yet.

### Describe the bug

I see cases where the model would return a result that includes the string ..."<|end|><|user|>"...
Looks like its breaking out of the <|assistant|> token instructions.

It would be good if next version would allow
1. Better support of specifying stop token
2. Support for canceling a generation

#2 would allow to implement our own stop sequence in a streaming response.

### Steps to reproduce the bug

This took running the model at scale to detect

### Expected behavior

_No response_

### Screenshots

_No response_

### NuGet package version

None

### Packaging type

_No response_

### Windows version

_No response_

### IDE

_No response_

### Additional context

_No response_

Start by reproducing the Phi Silica generation behavior described in issue #5203, ideally at scale, and trace the streaming generation entry point that handles stop tokens. Done means the model no longer returns the reported token sequence and the requested stop-token and generation-cancellation behavior is supported, with tests covering both.

Written by the indexing model from the issue text.