[Prism] Connect Web UI cancel requests with backend

[damondouglas]

About

This PR addresses #29669 connecting Web UI cancel requests with the backend Job Management server.

Changes

• Fixes loading of CANCELED state for previously canceled Jobs
• Triggers Web request from UI to backend on cancel clicks
• Adds to /job/cancel http handler more detailed error reporting

Testing

In addition to go test ..., manual testing of this PR was as follows:

1. Run prism via go run ./go/cmd/prism
2. Execute example via go run ./go/examples/large_wordcount --output=/tmp/large_wordcount/out --runner=universal --endpoint=localhost:8073 --environment_type=LOOPBACK
3. Validate job display in web console at http://localhost:8074/job/job-001 with state RUNNING
4. Click Cancel button
5. Validate cancel-related log messages populate in console after filtering for debug level logs
6. Validate Cancel button disabled with Job state change from RUNNING to CANCELLING
7. On reload of http://localhost:8074/job/job-001, validate state is CANCELED
8. Navigating to http://localhost:8074 shows job-001 in Jobs list as CANCELED

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[lostluck]

LGTM, though I do have one comment.

[lostluck]

deferring unlock isn't a straightforward universal good.

So this locks the whole server's shared state until the Cancel is complete, while it's doing the per job changes. You would at best want to lock the job's state, not serialize everything the server is handling.

The various job state fields should be locked through a job specific lock, which I suppose might be job.streamCond.L. State in particular is an atomic to avoid needing a full job lock for every read. With cancelling that might need to change however.

The Prepare method definitely has some overzealous locking (it's doing a lot of work, but not a lot of mixing with the server side state.

[damondouglas]

Now it makes sense to me that state is an atomic.Value and thus I didn't need to put a lock on the whole service. I only lock/unlock when acquiring the Job just like all the other methods.