[integrations] Align StellarRoute quote and route JSON parsing with live v1 schema

[yinkscss]

Context

_unwrap_envelope and _parse_path assume flexible field names (path, routes, paths). StellarRoute API may return stricter OpenAPI shapes; mismatches yield empty paths and weak predictions.

Scope

• Document expected StellarRoute response fields with version note
• Harden parsing with explicit validation errors logged per pair

Acceptance criteria

• Parsing handles documented StellarRoute quote/route payloads
• Unhealthy signal when required fields missing
• Does not reimplement pathfinder (NG1)

References

• src/stellarhydra/integrations/stellarroute_client.py
• StellarRoute /api/v1/quote, /api/v1/routes (companion repo)