feat: heuristic response analysis + reputation fixes

[jorgecuesta]

Summary

• Add qos/heuristic package for tiered response analysis to detect errors in response payloads without full JSON parsing
• Fix backend 5xx reputation bug where successful relay with backend 5xx was recording "ok" reputation signal
• Wire heuristic detection to reputation system so payload errors affect endpoint scores

Changes

Heuristic Response Analysis (qos/heuristic/)

New package with tiered analysis approach:

• Tier 1 - Structural: Empty responses, HTML error pages, XML, non-JSON, malformed JSON (~224ns)
• Tier 2 - Protocol: JSON-RPC result vs error field detection, REST error patterns
• Tier 3 - Indicators: Blockchain-specific errors, rate limits, connection errors, auth failures

Backend 5xx Reputation Fix (protocol/shannon/context.go)

• handleEndpointSuccess now checks endpointResponse.HTTPStatusCode
• Backend 5xx records CriticalErrorSignal (-25) instead of success (+1)
• 4xx errors NOT penalized (often user's fault - bad requests, auth issues)

Gateway Heuristic-to-Reputation Integration (gateway/http_request_context_handle_request.go)

• checkResponseSuccess() returns heuristic result alongside success status
• recordHeuristicErrorToReputation() records correcting signals when heuristic detects payload errors
• High confidence (≥95%): MajorError (-10)
• Lower confidence: MinorError (-3)

Cleanup

• Removed unused hydrateLogger and hydrateLoggerWithPayload functions

Test plan

• All unit tests pass
• Linter passes
• Deploy to staging and verify:
  • Backend 5xx responses show critical_error reputation signal in metrics
  • Heuristic-detected errors show major_error or minor_error in metrics
  • Dashboard Mean Score correlates with Success %