feat(tvix/eval): contextful string coercion

String with contexts are always coerced to a string with the same
context.

Change-Id: I224814febd9cad196bb28876793e76bed564dc72
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10440
Tested-by: BuildkiteCI
Autosubmit: raitobezarius <tvl@lahfa.xyz>
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/vm/generators.rs

@@ -604,7 +604,7 @@ pub async fn request_string_coerce(
         _ => match co.yield_(VMRequest::StringCoerce(val, kind)).await {
             VMResponse::Value(Value::Catchable(c)) => Err(c),
             VMResponse::Value(value) => Ok(value
-                .to_str()
+                .to_contextful_str()
                 .expect("coerce_to_string always returns a string")),
             msg => panic!(
                 "Tvix bug: VM responded with incorrect generator message: {}",