node: Fix race condition with accessing stdioOptions

[anaisbetts]

What does this PR do?

At Anthropic, we are using Bun for an internal CLI-based project that makes fairly significant use of node's child_process module. Intermittently across both macOS and Linux, we would see a crash in exec / spawn with a stack similar to:

TypeError: undefined is not an object (evaluating 'this.#stdioOptions')
      at #getBunSpawnIo (node:child_process:509:37)
      at <anonymous> (node:child_process:582:47)
      at #createStdioObject (node:child_process:566:27)
      at <anonymous> (node:child_process:591:57)
      at <anonymous> 

We believe that this is a race condition / ordering issue:

1. The onExit callback accesses this.stdio if hasSocketsToEagerlyLoad is true
2. this.stdio getter calls #createStdioObject
3. #createStdioObject uses #getBunSpawnIo
4. #getBunSpawnIo tries to access this.#stdioOptions

But at this point, this.#handle hasn't been set yet - it's set right after accessing stdio. So if the process exits very quickly, the onExit callback could run before the handle is properly set up, leading to this error.

This is particularly likely when:

• The spawned process exits immediately
• There are 3 or more stdio descriptors (which makes hasSocketsToEagerlyLoad true)

This change ensures that:

1. We first set this.#handle = handle and update the pid
2. Only then do we access this.stdio if needed
3. Finally schedule the #handleOnExit callback

This should prevent the race condition where stdio is accessed before the handle is set, which was causing the TypeError: undefined is not an object (evaluating 'this.#stdioOptions') error

• Documentation or TypeScript types (it's okay to leave the rest blank in this case)
• Code changes

How did you verify your code works?

Because this is a fairly tight race condition, it is difficult to verify outside of certain machines. We have a coworker who for whatever reason would hit this issue every time, and this patch fixes it. The full release test suite was also run locally and child_process tests all pass.

[nektro]

approved CI to start