feat: add feature toggle for incremental HTTP retry backoff [WPB-23381]

[screendriver]

WPB-23381 [Web] Make sure that the web app applies incremental backoff on 420/429

Pull Request

Summary

Introduce the startup feature toggle incremental-http-retry-backoff so the new HTTP retry policy can be rolled out safely behind an explicit client-side gate. This keeps current transport behavior unchanged while preparing the app for a gradual, reversible backoff rollout.

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Security Checklist (required)

• External inputs are validated & sanitized on client and/or server where applicable.
• API responses are validated; unexpected shapes are handled safely (fallbacks or errors).
• No unsafe HTML is rendered; if unavoidable, sanitization is applied and documented where it happens.
• Injection risks (XSS/SQL/command) are prevented via safe APIs and/or escaping.

Accessibility (required)

• I have read and this PR upholds our Accessibility Best Practices.

Standards Acknowledgement (required)

• I have read and this PR upholds our Coding Standards and Tech Radar Choices.

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Screenshots or demo (if the user interface changed)

Notes for reviewers

• Trade-offs:
• Follow-ups (linked issues):
• Linked PRs (e.g. web-packages):

[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

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🧪 Playwright Test Summary

• ✅ Passed: 11
• ❌ Failed: 0
• ⏭ Skipped: 3
• 🔁 Flaky: 0
• 📊 Total: 14
• ⏱ Total Runtime: 530.1s (~ 8 min 50 sec)