mcp: validate tool names (#640)

Implement SEP 986, enforcing restrictions on tool names. Since we're
already at v1 we couldn't turn this into a panic, so use an error log
instead. This is consistent with the typescript SDK. A note is left in
our 'rough edges' doc to panic in v2.

Fixes #621

internal/docs/rough_edges.src.md

@@ -9,3 +9,9 @@ v2.
   of the SDK where event persistence and delivery were combined.
   
   **Workaround**: `Open` may be implemented as a no-op.
+
+- Enforcing valid tool names: with
+  [SEP-986](https://github.com/modelcontextprotocol/modelcontextprotocol/issues/986)
+  landing after the SDK was at v1, we missed an opportunity to panic on invalid
+  tool names. Instead, we have to simply produce an error log. In v2, we should
+  panic.