When jQuery.when() is called with a single pending jQuery Deferred, the returned
promise now resolves synchronously (via .done()/.fail()) instead of being
scheduled asynchronously via setTimeout through .then().

The Problem

As reported in #4798, jQuery.when(deferred) unnecessarily introduces asynchronous
behavior when the single argument is a jQuery Deferred:

const deferred = jQuery.Deferred();
const promise = jQuery.when(deferred);
promise.done(() => console.log("done"));
console.log("before");
deferred.resolve();
console.log("after");
// Before fix: "before", "after", "done" (async via setTimeout)
// After fix:  "before", "done", "after" (synchronous)

This happens because jQuery.when returns primary.then() for pending single-value
cases, and .then() schedules handlers via window.setTimeout per the Promises/A+
spec. However, jQuery Deferreds use .done()/.fail() for synchronous notification,
so this async scheduling is unnecessary when the argument is a Deferred.

The Fix

When the single argument has a .promise() method (i.e., is a jQuery Deferred),
pipe resolution through a subordinate Deferred using .done()/.fail() instead
of .then(). The resolved value is re-adopted via adoptValue to preserve
secondary thenable unwrapping (cf. gh-3000).

Non-jQuery thenables (native Promises, custom thenables) still use the .then()
path, preserving their expected async behavior.

Tests

Added a new test case "jQuery.when(pendingDeferred) - synchronous resolution (gh-4798)" that verifies:

• .done() handler fires synchronously on resolve
• .fail() handler fires synchronously on reject
• Resolved values are correctly propagated
• Multi-value resolution preserves all values
• Promise state updates synchronously

All existing tests pass, including the Promises/A+ compliance suite (872/872).

Fixes gh-4798