Per-session prompt tracking in vMCP

[jhrozek]

We discover and track capabilities (tools, resources, prompts) per session in vMCP. This is the right thing to do, but because mark3labs lacks support for per-session prompts, we never added per session prompt tracking to vMCP but forgot to add a fallback.

We should either grab prompts globally or extend the SDKs.

[yrobla]

Related to stacklok/toolhive-rfcs#38