refactor: derive routers from protocol metadata

[frostming]

Summary

Adding or changing an ACP method previously required updating both its Agent/Client protocol declaration and a separate router registration. Move the wire method name, request/notification kind, unstable flag, optional-handler defaults, and adapters into param_model / param_models on the protocols. Both router entry points now delegate to MessageRouter.from_protocol.

The common builder preserves legacy handler dispatch, extension methods, optional terminal responses, unstable gates, and separate request/notification handlers for mcp/message. Config-option and elicitation conversions live in a private adapter module and are selected by protocol metadata. Model-only decorators on outgoing connection methods remain compatible with signature generation and legacy calls. No schema regeneration is included.

Related issues

Follow-up to #148.

Testing

• Python 3.10 full suite, including transport extras: 312 passed, 2 skipped.
• Python 3.14 full suite: 313 passed, 1 skipped.
• make check: all lockfile, pre-commit, type, and dependency checks passed.
• Compared generated and previous router tables with unstable support both enabled and disabled: methods, kinds, flags, defaults, and result adapters match.
• Added tests for inherited declarations, protocol-only routing, union validation, duplicate-route rejection, legacy config/elicitation adapters, extensions, and preservation of decorator metadata during signature generation.

Docs & screenshots

Document route declarations and adapter options in the quickstart maintenance section.

Checklist

• Conventional Commit title (e.g. feat:, fix:).
• Tests cover the change or are not required (explain above).
• Docs/examples updated when behaviour is user-facing.
• Schema regenerations (make gen-all) are called out if applicable.