docs: update SSE fetchStream subscription example

[v0eak]

Summary

What — What changes are introduced in this PR?
Example on how to receive SSE events using JS fetchStream updated to reflect refactor

Why — Why are these changes relevant or necessary?
Reflect Refactor

Additional Context

Intended to be merged after: #15134

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[vercel]

@v0eak is attempting to deploy a commit to the medusajs Team on Vercel.

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[medusa-os-bot]

Thank you for your contribution, @v0eak!

Could you please fill in the PR description following our pull request template? We need the What, Why, How, and Testing sections completed so we can review your PR efficiently.

Once you've updated the description, another review will be triggered automatically.

Note: This PR removes the if (\!stream) null check based on the refactor in PR #15134, which is still open. Please clarify in the description whether this PR is intended to be merged after #15134, so the team can coordinate the merge order and avoid shipping documentation that describes behavior not yet in the codebase.

Thanks!

Triggered by: new PR opened

[medusa-os-bot]

Thank you for your contribution, @v0eak!

After reviewing the updated description, we're still missing a couple of sections before we can move forward:

Required changes:

• Fill in the How and Testing sections of the PR description — the previous review requested all four sections of our pull request template, and these two sections are still absent.

Once you've updated the description, another review will be triggered automatically.

Thanks!

Triggered by: PR description updated

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