solution: wire SolutionServer into agents.Serve() so the contract is servable

[antoinetoussaint-byte]

Follow-up from #287 (see PR #288 review, finding D4).

The Solution gRPC contract exists and is generated, but a solution plugin has no way to expose it through the standard agent startup path: agents/agents.go's PluginRegistration has no Solution solutionv0.SolutionServer field and Serve() does not register one (compare the Provider providerv0.ProviderServer field + providerv0.RegisterProviderServer call).

• Add Solution solutionv0.SolutionServer to PluginRegistration.
• Register it in Serve() (if reg.Solution != nil { solutionv0.RegisterSolutionServer(s, reg.Solution) }).
• Cover it with an agents registration test.

Blocks the end-to-end tracer bullet (obin-ai/lodestar#17) — a solution binary cannot serve its contract until this lands.