[in-progress] feat: Update CBOR protocol

[jterapin]

Issue

Follows #348 (generic Protocol plugin, define-only). That PR added the mechanism but nothing consumed it. CBOR was still served by a per-service plugin (Plugins::RpcV2Cbor) with its serialization split across two handler classes and a separate stubber, none of which used the new interface.

Description

Retrofit RPC v2 CBOR onto the generic Protocol plugin, making it the first real consumer and taking the mechanism live:

• One protocol object. A standalone Client::RpcV2Cbor replaces the old plugin, its two handlers (Handler + ErrorHandler), and the separate CBOR stubber, implementing the 5-method interface (build_request / parse_data / parse_error / stub_data / stub_error).
• Interface cleanup. Drop the Protocol base class; the interface is now duck-typed and documented on the standalone NoOpProtocol, which also carries the stubbing methods. The generic ParseHandler passes context (not the bare response) so a protocol can reach request headers, operation errors, and the client's errors module.
• Stubbing through the protocol. Stubbing routes through config.protocol instead of a separate :stubber option; the protocol object doubles as its own stubber.
• Real registry. A new add_protocols weld hook populates client.protocols at codegen, replacing the test-only registry scaffold from feat: Add generic Protocol plugin and protocol interface #348.

parse_error reproduces the old ErrorHandler's on_done(200..599) gate with an early return, so signaled transport errors (status 0) pass through untouched.

Testing

• smithy-client: 858 examples, 0 failures, 2 pending
• smithy unit: 505, 0 failures
• generated CBOR protocol tests: 113, 0 failures
• RuboCop clean

Note: AWS protocols still use a String :protocol + a separate :stubber and are migrated in a follow-up PR.

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