[JENKINS-75793] Fix inconsistent rendered values when using multiple reactiveChoice parameters

[baczus]

This PR fixes issue #902.

Root cause:

In the makeStaplerProxy2 function (introduced in v2.8.4 when PR #79 was reverted), the .then() handler inside the fetch call was missing return statements before response.json().then(...) and response.text().then(...). Without those returns, the outer await resolved as soon as the HTTP response headers arrived, before the response body was parsed and the callback was invoked. This broke the serialization of cascade parameter updates -- when rapidly changing env, the doUpdate and getChoicesForUI calls for chained parameters (stream, commit) could overlap, causing stale values (e.g., dev_commit1 showing up when prod was selected).

Fix:

Added return before both response.json().then(...) and response.text().then(...) in makeStaplerProxy2, so the promise chain properly propagates and await blocks until the callback completes.

Testing done

I tested the fix locally using the test job provided in the original issue, against the latest Jenkins LTS version.

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