[Bug]: Add validation for tool requests that the tool is available #29432
Description
🐛 Describe the bug
Currently there is no check for this, vLLM will just respond sub-optimally in a "silent" way. We should fail the individual request in this case with appropriate error message.
This is a problem in particular for example when using the demo tool server without the gpt-oss package installed.
See discussion here: #29336 (comment)