Update registration details for GitHub certifications exam

[sammiekiogora]

Updating details related to proctor change (PSI to Pearson Vue)

Why:

Closes:

What's being changed (if available, include any code snippets, screenshots, or gifs):

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• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

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Source	Review	Production	What Changed
get-started/showcase-your-expertise-with-github-certifications/registering-for-a-github-certifications-exam.md	fpt ghec	fpt ghec

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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[Copilot]

Pull request overview

Updates the GitHub Certifications exam registration article to reflect the proctoring provider change from PSI to Pearson VUE and adjusts scheduling guidance accordingly.

Changes:

• Replaces PSI-specific scheduling language with Pearson VUE details and links.
• Updates the “online vs. test center” guidance with system pre-check and support resources.
• Adds/adjusts scheduling-window policy language for Pearson VUE.