fix(recipes): correct error_descriptor/error_parts field names to match API

[Abhishek-Bhatta]

Summary

Follow-up to the review on #93 — targets zt/api-gap-closure directly (per @zaynelt's request in this comment) rather than develop, since it's a correction on top of that PR's existing structs.

ErrorDescriptor and ErrorParts declared json tags (type, message) that don't exist in the real Workato API response, so both structs silently decoded to their zero value on every real job — verified against a live dev API job (GET /recipes/{id}/jobs/{job_id}):

"error_descriptor": {"error_type":"Exception","error_id":"35601e00-...","line_number":null,"adapter":"Workato","error_at":"...","actionable":true,"action":null,"trigger":null}
"error_parts": {"message":"...","error_type":"Exception","error_id":"35601e00-...","action":"make_request_v2","line_number":6,"adapter":"rest","retry_count":0}

This PR:

• Corrects both structs' field names/types to match the real schema
• Expands HTTPResponse (protocol, normalized_status_text) and ErrorDetails (message, inner_message) to match the full response
• Registers all four structs (ErrorDescriptor, ErrorDetails, HTTPResponse, ErrorParts) in TestStructFieldCoverage — none of them were covered before, which is how the mismatch went unguarded
• Extends TestRecipeService_GetJob to assert on the real field names

Verified end-to-end against a live failed job — error_descriptor and error_parts now populate correctly instead of decoding empty.

Checklist

• go build ./... and go test ./... pass
• Docs / CHANGELOG.md updated as needed — not touched; scoped as a correction to unreleased structs from Zt/api gap closure #93, no user-facing behavior to log yet
• N/A ADR check — no ADR covers this

[zaynelt]

Thanks @Abhishek-Bhatta! Really appreciate you catching the omissions & errors and contributing the correct shapes.