feat(builtins): support Symbol unregister tokens in FinalizationRegistry

[tkshsbcue]

Summary

Implement CanBeHeldWeakly checks per ECMAScript spec for FinalizationRegistry.prototype.register and .unregister methods.

Non-registered symbols (created via Symbol()) are now accepted as unregister tokens, while registered symbols (Symbol.for()) correctly throw TypeError.

Changes

• Add UnregisterToken enum to hold either Object or Symbol tokens
• Add is_registered_symbol() helper in symbol module
• Update register() to accept symbol unregister tokens
• Update unregister() to match against symbol tokens
• Add tests for symbol token usage and registered symbol rejection

How it works

The CanBeHeldWeakly abstract operation returns true when:

1. The value is an Object, OR
2. The value is a Symbol and KeyForSymbol(v) is undefined (i.e., not created via Symbol.for())

Previously, only objects were accepted as unregister tokens. Now symbols work too:

const registry = new FinalizationRegistry(() => {});
const sym = Symbol("token");

registry.register({}, undefined, sym);  // works now
registry.unregister(sym);               // works now

registry.register({}, undefined, Symbol.for("x"));  // throws TypeError (registered symbol)
registry.unregister(Symbol.for("x"));                // throws TypeError (registered symbol)

Limitations

Symbol targets (first argument to register()) are not yet supported as they require Ephemeron support for non-GC types. This is tracked separately and can be addressed in a follow-up PR.

Test plan

• Symbol as unregister token: register and unregister via Symbol("token") works correctly
• Registered symbol rejection: Symbol.for("x") throws TypeError for both register and unregister
• All 4 existing tests still pass unchanged

Closes #5253

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	50,732	50,732	0
Ignored	1,426	1,426	0
Failed	805	805	0
Panics	0	0	0
Conformance	95.79%	95.79%	0.00%

Tested main commit: 2437b6704c4315f6ec8f0766bf229fba23e0e61e
Tested PR commit: a89b4794569c6f5c366232c9655b602931f501a8
Compare commits: 2437b67...a89b479

[codecov]

Codecov Report

❌ Patch coverage is 72.41379% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.73%. Comparing base (6ddc2b4) to head (a89b479).
⚠️ Report is 919 commits behind head on main.

Files with missing lines	Patch %	Lines
...e/engine/src/builtins/finalization_registry/mod.rs	70.37%	8 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5259       +/-   ##
===========================================
+ Coverage   47.24%   59.73%   +12.48%     
===========================================
  Files         476      589      +113     
  Lines       46892    63424    +16532     
===========================================
+ Hits        22154    37886    +15732     
- Misses      24738    25538      +800     

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[jedel1043]

This is doing things backwards. We should first add support for JsSymbol in our Gc and only then add this feature.