fix(ten_rust): return an error from GraphLoc::check_node_exists for a location with no node

[RaphaelFakhri]

Summary

GraphLoc::check_node_exists called unwrap() on get_node_name() and get_node_type(). Both return an error when the location has no extension, subgraph or selector. In the tman designer, adding a connection without selecting a node reaches this function and the server panics with the type of the node must be one of extension/subgraph/selector.

The function returns Result, so this change propagates the error with ?. The API caller receives an error instead of a crashed worker.

Fixes #1877

Test

Adds graph::loc to the ten_rust integration tests. It calls check_node_exists on an empty GraphLoc and asserts the returned error message.

• Without the fix, the test panics at src/graph/connection/mod.rs:170 with the message from the issue.
• With the fix, the test passes, and the other 84 graph:: tests still pass.

cd core/src/ten_rust
cargo test --test integration_test graph::