markupMessageSupport is hard-coded to false, what would it take to enable it? #1860

DiagnosticFeature.fillClientCapabilities sets:

// VS Code has no support for markup content in diagnostic messages.
capability.markupMessageSupport = false;

(client/src/common/diagnostic.ts, ~L1141 on main)

I want to request functionally to support MarkupContent diagnostic messages with unified-language-server (remark-based markdown linting).
It's messages are authored as markdown at the source, so today things like Don't add a trailing `:` to headings reach the user with the backticks intact.

Two questions:

1. Is that flag waiting purely on feature: Allow diagnostics messages to have markdown content vscode#214051 / Allow diagnostics messages to have markdown (or formatted text) content vscode#54272
   landing markdown support in vscode.Diagnostic.message, or is there
   something else blocking it on the client side?

2. What should a push-only server read?
   The 3.18 guard is textDocument.diagnostic.markupMessageSupport, which is a pull-model
   capability and PublishDiagnosticsClientCapabilities has no equivalent.
   But asDiagnostic (client/src/common/protocolConverter.ts, ~L329) is shared
   between the push and pull paths and switches on the value type rather than the negotiated capability.
   Zed hit the same thing in lsp: Add client support for LSP 3.18 markdown diagnostic messages zed-industries/zed#61030 and advertises the capability only when pull
   diagnostics are enabled, while handling markup on both paths.

Happy to open a spec issue instead if question 2 belongs upstream.