fix(server): surface chat-template errors

[Odysseusailoon]

📋 PR Title

fix(server): surface chat-template errors

📝 Change Type

• fix: Bug fix.

💡 Description

Malformed chat requests that embed <|channel|> tags in messages[*].content caused the executor’s tokenizer step to raise a template error, leaving the scheduler hanging and never responding to the client. This PR forwards those failures back through the IPC channel so the HTTP handler can immediately return a structured 400 error while keeping the node healthy. It also adds unit coverage for the new HTTP error-handling path.

Key Changes

1. Add _notify_http_request_error in Executor to catch tokenizer/chat-template exceptions and send error envelopes to the HTTP server.
2. Extend HTTPHandler to track per-request error state, stream error chunks, and emit non-streaming 400 responses.
3. Manually test it on the machine.

🔗 Related Issues

• Closes [Bug]: Failed requests cause the node to be unavailable #158

[Odysseusailoon]

123