feat: Update hometest supplier spec to add app-level3 auth and new integration environment

[iichr]

Description

Update spec before publishing to UAT.

1. Replaced the generic bearer authentication scheme with APIM's app-level3 application-restricted signed-JWT security scheme (following the example of other OASs)
2. Documented that suppliers authenticate their application, not an end user, and linked to the NHS signed-JWT integration guide.
3. Added the sandbox, integration and production environment table. Only the sandbox URL is currently available; integration and production remain unavailable.
4. Corrected the sandbox documentation to state that it returns canned responses and enabled the "Try this API" feature.

Context

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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Sensitive Information Declaration

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• I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud