[Serve] Make long poll wait for non-existent keys

[simon-mo]

Why are these changes needed?

This makes long poll host wait for non-existent keys instead of immediately erroring and trigger client to immediately retries. The original issue happened because a deployment takes a long time to startup, but http proxy already created the handles for those deployments and tries to pull the REPLICA_HANDLES for the deployments. The handles key doesn't exist as deployment pending, so long poll client keep retries and cause a lot of exceptions (key not found) in controller and it being retried every 50ms or so.

Related issue number

Closes #19201

Checks

• I've run scripts/format.sh to lint the changes in this PR.
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• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
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