fix: don't swallow rejections in poll_value()

[nayeemrmn]

Unblocks denoland/deno#20082.

Fixes three tangled issues:

• JsRuntime::poll_value(promise) swallows non-awaited rejections returned from poll_event_loop() when promise is already settled.
• JsRuntime::poll_value(promise) runs the event loop once even if promise is already settled.
• JsRuntime::call_and_await() needs to explicitly call check_promise_rejections() at the end.

Some test expectations might need to add (in promise) to error strings. These are correct. There's at least one in CLI repo:

diff --git a/cli/tests/testdata/node/test.out b/cli/tests/testdata/node/test.out
index 8b7f0780f..3c54a15e8 100644
--- a/cli/tests/testdata/node/test.out
+++ b/cli/tests/testdata/node/test.out
@@ -147,7 +147,7 @@ error: Error: rejected from reject fail
     at [WILDCARD]
 
 ./node/test.js (uncaught error)
-error: Error: rejected from unhandled rejection fail
+error: (in promise) Error: rejected from unhandled rejection fail
   Promise.reject(new Error("rejected from unhandled rejection fail"));
                  ^
     at [WILDCARD]

[bartlomieju]

@nayeemrmn what do you mean by

After fixing these, JsRuntime::call_and_await() doesn't account for non-awaited rejections.

How that should be handled by the embedder?

[nayeemrmn]

@nayeemrmn what do you mean by

After fixing these, JsRuntime::call_and_await() doesn't account for non-awaited rejections.

How that should be handled by the embedder?

I wrote it confusingly. It was a third issue introduced by fixing the first two but it's also fixed.

[bartlomieju]

LGTM!