fix(SR): Bring back the onModeEnter for the cornerstone-dicom-sr extension that was accidentally removed by PR #4586

[jbocce]

Context

The title says it all. See #4586.

Changes & Results

Testing

SRs should load.

Checklist

PR

• My Pull Request title is descriptive, accurate and follows the
  semantic-release format and guidelines.

Code

• My code has been well-documented (function documentation, inline comments,
  etc.)

Public Documentation Updates

• The documentation page has been updated as necessary for any public API
  additions or removals.

Tested Environment

• OS: Windows 11
• Node version: 18.16.1
• Browser: Chrome 131.0.6778.140

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