[2.x] fix(js): capture extension name by value in initializer error closure

[imorland]

Summary

Fixes #4293.

When an initializer throws, the error is deferred into a caughtInitializationErrors closure and fired after mount() completes. The closure was reading this.currentInitializerExtension at call time — by which point the property holds the last initializer's name, not the one that threw. So every error message incorrectly blamed the final extension in the boot sequence.

The fix is a one-liner: capture this.currentInitializerExtension into a local const inside the catch block before creating the closure, so each closure closes over its own immutable value.

extend.ts already does this correctly (const extension = app.currentInitializerExtension at line ~34). The catch block was just not updated to match when currentInitializerExtension was introduced in #4134.

Test plan

• New integration test: registers two initializers where the first throws, calls initialize(), and asserts the error closure names bad-ext (the throwing extension), not good-ext (the last one to run)
• Run yarn test in framework/core/js/