view: add #[diagnostic::on_unimplemented] hint to HasMessageView

[iainmcgin]

Summary

Adds a #[diagnostic::on_unimplemented] hint to buffa::HasMessageView, mirroring the existing ViewEncode diagnostic in the same module.

HasMessageView impls are emitted by codegen next to each message's view types (since 0.7.0). Downstream consumers bound on it in generated code — most prominently connect-rust's service stubs, where the unsatisfied bound surfaces inside the consumer's generated file at a call site they cannot annotate. With this change the error explains the cause and the fix instead of only naming the missing trait bound:

error[E0277]: `Simple` does not implement `HasMessageView` — its message-view family was not generated or is not enabled
  = note: the `HasMessageView` impl is emitted next to each message's view types: regenerate the crate that
          defines `Simple` with buffa 0.7.0 or newer and views enabled — `generate_views(true)` (on by default)
          in a buffa-build / buffa-codegen config, or `views=true` for protoc-gen-buffa
  = note: if the defining crate feature-gates its generated impls, enabling its views feature is enough — no
          regeneration needed

(Rendered output verified against a views-off generated type from buffa-test.)

Notes

• #[diagnostic::on_unimplemented] stabilized in Rust 1.78; MSRV is 1.87, and ViewEncode already carries the same attribute form.
• No test added: there is no compile-fail/UI test harness in the repo, and the attribute has no runtime behavior; the rendered output was verified manually as above.
• CHANGELOG entry added under Unreleased → Changed.

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