feat: observability rate limiting error envelope

[rachealkenny]

Implement request correlation propagation, standardized error envelope, and configurable rate limiting

Overview

This PR adds request correlation propagation for distributed tracing, standardizes error responses across all controllers, and implements configurable rate limiting per endpoint group.

Changes

Request Correlation Propagation (#557)

• Propagate correlation IDs to AI-service, onchain adapter, and all external HTTP calls
• Ensure correlation ID appears consistently in all logs for traceability

Standardized Error Envelope (#556)

• Implement consistent error envelope across all endpoints: { code, message, correlationId, details }
• Align existing endpoints to the new format
• Add E2E tests validating at least 3 representative error cases

Rate Limit Policy Map (#551)

• Make rate limiting configurable per endpoint group (public, admin, webhooks)
• Centralize configuration with sane defaults and environment-aware overrides for testnet
• Add tests covering guard behavior on key endpoints

Files Added

• Correlation ID propagation middleware
• Error envelope formatter and exception handler
• Rate limit configuration map

Files Modified

• HTTP client for outbound calls
• All controller error responses
• Rate limiting middleware

Closes #557
Closes #556
Closes #551

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