Fix Proxy [[OwnPropertyKeys]] check for integer-index keys (#1609)

[tangtaizong666]

Summary

Reflect.ownKeys / Object.keys / Object.getOwnPropertyNames on a Proxy
whose target is non-extensible and has an integer-index own key (e.g.
"12345") throws

TypeError: ownKeys target is non-extensible but key is missing from trap result

even when the trap faithfully returns the target's own keys. This breaks common
state libraries such as Valtio (see #1609), where a frozen snapshot object with
numeric string keys can no longer be enumerated.

Root cause. JSProxy::ownPropertyKeys enforces the ES [[OwnPropertyKeys]]
invariant (steps 19 and 21) by checking that every own key of the target is
present in the trap result using SameValue. Hermes represents integer-index
own-property keys internally as numbers (JSObject::getOwnPropertyKeys
returns 12345 as a Number), whereas the trap result is validated to contain
only Strings and Symbols and therefore holds the key as the String "12345".
SameValue(12345, "12345") is always false, so the invariant check fails and
throws. The comparison only runs for non-extensible targets, which is why the
bug only surfaces once the target is frozen / preventExtensions'd.

Fix. Before the invariant comparisons, normalize any numeric index keys in
targetKeys to their canonical string form — the same conversion that for-in
enumeration already performs for numeric array indices
(Interpreter-slowpaths.cpp, "We must return the property as a string"). The
returned key list is unchanged (it still comes from the trap result), so this
only removes the spurious invariant failure.

Test Plan

New regression test test/hermes/regress-proxy-ownkeys-numeric-key.js covering:

• an integer-index key on a preventExtensions target (configurable-keys loop),
• integer-index keys on a Object.freezed target (non-configurable-keys loop),
• mixed integer/string key ordering, and
• integer-index keys alongside symbols.

$ ./build-rel/bin/hermes -Xes6-proxy -non-strict -O -target=HBC \
    test/hermes/regress-proxy-ownkeys-numeric-key.js
case1: 12345
case2: 0,42
case3: 1,2,b,a
case4: 7,Symbol(s)

The test passes with this change and fails on the unpatched engine with the
reported TypeError. Existing test/hermes/proxy.js continues to pass.

Fixes #1609