Fix ArrayStoreException in ObjectArrays.concat(element, array) / concat(array, element)

[Junnie18]

Summary

ObjectArrays.concat(T element, T[] array) and ObjectArrays.concat(T[] array, T element) build their result array with the same runtime component type as the input array (via newArray/Arrays.copyOf), then store element directly into it. If element's runtime type isn't assignable to that component type — which erasure allows the caller to do, since both methods are generic — storing it throws ArrayStoreException instead of succeeding:

String[] array = new String[0];
Object[] concatenated = ObjectArrays.<Object>concat(array, 0); // throws ArrayStoreException

This is inconsistent with ObjectArrays.concat(T[], T[], Class<T>), which never has this problem because it always builds its result array from the explicitly supplied Class<T> rather than inferring a component type from one of its array arguments.

Fix

Each of the two single-element overloads now checks whether element is storable in an array with the same component type as the input array before allocating the result that way. If it isn't, it falls back to a plain Object[] (unchecked-cast back to T[]), which mirrors the existing unsoundlyCovariantArray pattern already used elsewhere in this file for the same class of unsoundness. In the overwhelmingly common case (element and array are actually type-compatible), behavior and the returned array's runtime type are unchanged.

No public API was changed.

Testing

Added 4 regression tests to ObjectArraysTest (prepend/append, with both an empty and a non-empty backing array) that concatenate a String[] with an Integer via an explicit <Object> type witness, reproducing the exact shape of bug report's repro. Verified:

• Before the fix: all 4 new tests fail with ArrayStoreException, confirming they exercise the reported bug.
• After the fix: mvn test -pl guava,guava-tests -am -Dtest=ObjectArraysTest → 30/30 pass.
• Ran the full com.google.common.collect.*Test suite (207 test classes) afterward to check for regressions: all pass.
• Formatted the changed source file with google-java-format.

Fixes #3768.

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[ionik0]

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