fix(router): persist and apply retry_policy via update_settings (#31308)

[shivshekhar12]

Summary

retry_policy set in the Admin UI / config was silently dropped:

• It was missing from UpdateRouterConfig, so Pydantic dropped it at parse time before the DB write.
• It was missing from the Router.update_settings() allowlist, so a value loaded from the DB was never applied to the live router.

This PR:

• Adds retry_policy: Optional[RetryPolicy] to UpdateRouterConfig (moved RetryPolicy above it so it can be referenced).
• Adds retry_policy to the update_settings() allowlist.
• Coerces the on-the-wire dict into a RetryPolicy instance before applying it to the live router.

Test plan

• test_update_settings_retry_policy — Router.update_settings() accepts a retry_policy dict and coerces it into RetryPolicy.
• DB-load path applies retry_policy to the live Router on startup.
• POST /config/update persists retry_policy into the DB merged with existing values.
• UpdateRouterConfig parses and serializes retry_policy correctly.

Fixes BerriAI#31308

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