Normalize error responses across all tools

[esgn]

Description

Error responses are currently inconsistent across tools depending on where the failure happens:

• schema validation may expose raw Zod 3 messages / issue arrays
• some tools return curated domain-specific messages
• upstream service / HTTP failures are often passed through with minimal normalization

This makes the MCP error contract harder to consume, test, and document.

Scope

• keep existing upstream service behavior unchanged
• no Zod 4 migration
• improve only the errors exposed by geocontext

Expected outcome

• consistent error wording and structure across all tools
• no raw Zod issue dumps exposed to clients
• clearer classification of upstream transient / service errors
• preserve useful backend details when relevant

Non-goals

• changing upstream service responses
• migrating from Zod 3 to Zod 4