fix: set DEFAULT_CONTEXT_LENGTH to 8192

[pppy2012]

Change the DEFAULT_CONTEXT_LENGTH from 8096 to 8192, the reason is to enhance compatibility between continue and ollama to improve performance.
Key reasons:

• Ollama uses 8192 as the default context length.
• Ollama will reload the model if the context changes even the model is same.
• The ollama backend may reload models to support continue's requests with 8096 context length.
• Likewise, models preloaded for ​continue may be evicted by other requests using the default context length.
• Frequent model reloading introduces significant performance overhead.

So, it's better to use same default context length to improve the performance in default environment.

[netlify]

✅ Deploy Preview for continuedev canceled.

Name	Link
🔨 Latest commit	f163768
🔍 Latest deploy log	https://app.netlify.com/sites/continuedev/deploys/67edf5d6c1c10d0008f4b094