[Bug]: Add validation for tool requests that the tool is available

[njhill]

🐛 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)

[chaunceyjiang]

/cc @yeqcharlotte @qandrew

[AryanBagade]

@njhill if its open for external contributors, can i work on this?

[emanuelediluzio]

Hey! I'd love to take this as my first vLLM contribution if it's still free 🙂