add default retry policy for nango proxy calls

[devin-ai-integration]

Summary

Sets sensible default retry configuration on NangoClient::for_connection() so all proxy requests automatically leverage Nango's server-side retry mechanism. This addresses Nango's rate limit guidance by retrying on 429 (and common server errors) up to 3 times via Nango's built-in Retries/Retry-On proxy headers — no new dependencies needed.

Currently for_connection() has no production callers (only test usage), so this ensures the right defaults are in place before proxy calls are wired up.

Review & Testing Checklist for Human

• Confirm the retry status code set (429, 500, 502, 503, 504) is appropriate — particularly whether retrying on 500 is desired for your proxy use cases
• Note that callers can still override via .retries(N) / .retry_on(...) if different behavior is needed per call site

Notes

• Devin run
• Requested by @yujonglee

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[devin-ai-integration]

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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