fix(github): improve callTool response handling

[tlgimenes]

Summary

• Handle result.isError by throwing with the text message or a fallback
• Return result.structuredContent directly when available
• Parse text content as JSON with explicit error on parse failure
• Add safeJsonParse helper for safe JSON parsing

Test plan

• Verify error responses from upstream MCP are properly thrown
• Verify structured content responses are returned directly
• Verify JSON text responses are parsed and returned
• Verify non-JSON text responses throw a clear error

🤖 Generated with Claude Code

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Summary by cubic

Improves callTool response handling in the GitHub MCP proxy. Throws on upstream errors, prefers structuredContent, and parses text responses as JSON with clear failures.

• Bug Fixes
  • Throw with the tool’s text message (or fallback) when result.isError is true.
  • Return result.structuredContent directly when available.
  • Parse text content as JSON; if parsing fails, throw Failed to parse: <msg>.
  • Add safeJsonParse helper and remove response logs.

^{Written for commit 67a5913. Summary will update on new commits.}