fix(promise): set [[PromiseIsHandled]] correctly

[ashnaaseth2325-oss]

Summary

This PR fixes how [[PromiseIsHandled]] is set in Promise::perform_promise_then.
According to the ECMAScript spec, step 12 of PerformPromiseThen must run unconditionally.
Currently, Boa sets handled = true only inside the Rejected branch, which means pending promises with a .catch() attached can still be reported as unhandled later.

Example of the previous placement:

PromiseState::Rejected(ref reason) => {
    // ...
    promise.borrow_mut().data_mut().handled = true;
}

Because of this, promises that register a handler while still pending may incorrectly trigger the host rejection tracker.

Steps to Reproduce

Example JavaScript:

const p = new Promise((_, reject) => {
  Promise.resolve().then(() => reject("oops"));
});

p.catch(() => {});

Expected behavior:
-The rejection is handled, so no unhandled-rejection tracking should occur.

Actual behavior before this fix:
-The host rejection tracker may fire because [[PromiseIsHandled]] was never set while the promise was pending.

Impact

Handled rejections can be reported as unhandled.
Hosts relying on HostPromiseRejectionTracker may emit incorrect warnings or test failures.
This mostly affects the common case where .catch() is attached before the promise settles.

Fix

Move the assignment of [[PromiseIsHandled]] so it runs after the match block, ensuring it applies regardless of the promise state.

// Step 12. Set promise.[[PromiseIsHandled]] to true.
promise.borrow_mut().data_mut().handled = true;

This aligns the implementation with ECMAScript §27.2.5.4.1 PerformPromiseThen step 12.

Result

-[[PromiseIsHandled]] is now set correctly for pending, fulfilled, and rejected promises.
-Handled rejections no longer trigger spurious unhandled-rejection tracking.
-Behavior now matches the ECMAScript specification.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	49,713	49,713	0
Ignored	2,262	2,262	0
Failed	988	988	0
Panics	0	0	0
Conformance	93.86%	93.86%	0.00%

Tested main commit: dc02d4f354ac278dd845f529a3331001c4ea0ede
Tested PR commit: 2c0cd6df030e07d4e2bac06101b029270ae20011
Compare commits: dc02d4f...2c0cd6d

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.36%. Comparing base (6ddc2b4) to head (2c0cd6d).
⚠️ Report is 800 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #4983       +/-   ##
===========================================
+ Coverage   47.24%   58.36%   +11.12%     
===========================================
  Files         476      556       +80     
  Lines       46892    61154    +14262     
===========================================
+ Hits        22154    35695    +13541     
- Misses      24738    25459      +721     

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[jedel1043]

Good catch!