feat: configurable service name

[franklinkim]

Description

Add a serviceNames target option to override the display name used for OTel spans and metrics per service.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 💥 Breaking change
• 📝 Documentation
• ♻️ Refactoring
• ⚡ Performance
• ✅ Tests
• 🔧 Build/CI

Changes

• Add serviceNames map[string]string to config.Target plus a Target.ServiceName(service) helper that falls back to the service name when no override is set (does not affect generated Go types or routing).
• Wire config.ServiceName(...) into renderTSRPCServiceProxies, renderTSRPCServiceClients, renderGoRPCServiceProxies, and renderGoRPCServiceClients so the telemetry name flows into NewServer / WithClientService.
• Extend gotsrpc.schema.json and gotsrpc.example.yaml with the new serviceNames property; document it in docs/guide/configuration.md.
• Harden updateCode in build.go: propagate the os.ReadFile error instead of ignoring it, tighten MkdirAll perms to 0700, and drop the //nolint:gosec suppressions.
• Clarify semconv.Config field docs for the client-side Package/Service usage.
• Regenerate the monitor example with serviceNames: { Service: Monitor }.

Checklist

• My code adheres to the coding and style guidelines of the project.
• I have performed a self-review of my own code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.