fix: corrected misleading storeValue logic in JS best practices

[YasasBanuka]

Description

Refactored the example code in "Use Appsmith store" (Best Practices) to separate the getter and setter logic.

• The previous example caused a side effect where fetching the name inadvertently reset it to the default value.
• fetch_name is now a pure getter that uses a fallback (|| 'sam').
• update_name handles the actual state change.

Pull request type

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• Review Fixes
• Documentation Overhaul
• Feature/Story
  • Link one or more Engineering Tickets
    • • A-Force
• Error in documentation
• Maintenance

Documentation tickets

Link to one or more documentation tickets:

• Closes [Docs]: Misleading logic in "Use Appsmith store" example (fetch_name) #2972

Checklist

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• Checked for Grammarly suggestions.
• Adhered to the writing checklist.
• Adhered to the media checklist.
• Verified and updated cross-references or added redirect rules.
• Tested the redirect rules on deploy preview.
• Validated the modifications made to the content on the deploy preview.
• Validated the CSS modifications on different screen sizes.

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