LSP 3.18 allows servers to send [`MarkupContent` diagnostic
messages](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.18/specification/#diagnostic),
guarded by the client capability
`textDocument.diagnostic.markupMessageSupport`.

- Advertise `markup_message_support` only while pull diagnostics are
enabled because `textDocument.diagnostic` is the pull-model capability.
  LSP 3.18 currently has no equivalent capability for push diagnostics:

https://microsoft.github.io/language-server-protocol/specifications/lsp/3.18/specification/#diagnosticClientCapabilities
  https://github.com/microsoft/language-server-protocol/issues/2246
- Handle `string | MarkupContent` messages in both push and pull
diagnostic paths: markdown-kind messages populate the existing
`Diagnostic::markdown` field (rendered in hover popovers and the
diagnostics panel), plaintext-kind markup remains plain text, and plain
string messages keep going through the adapter's
`diagnostic_message_to_markdown`.
- Preserve server-provided markup for code actions sent back to the
originating server, avoid forwarding markup diagnostics to other
servers, and render diagnostic metadata without altering the
server-provided markdown.
- Carry the markup kind over RPC via a new optional
`LspDiagnostic.markup_message_kind` proto field so remote/collab
projects round-trip `MarkupContent` messages.

Depends on the lsp-types change that turns `Diagnostic.message` into a
`DiagnosticMessage` enum (`string | MarkupContent`):
https://github.com/zed-industries/lsp-types/pull/15
Once merged, Cargo.toml can point to the upstream commit.

Note that Zed already rendered some plain-string diagnostics as Markdown
through language-specific adapter hooks, including `rust-analyzer`. This
adds protocol-level support for diagnostics explicitly sent as LSP 3.18
`MarkupContent`, while preserving the existing adapter fallbacks.

As a result, this does not introduce a new diagnostics rendering
pipeline.
Server-provided Markdown is routed through the existing
`Diagnostic::markdown` path, with small fixes to handle Markdown-only
updates and block content correctly.

## Showcase

Example from [JETLS.jl](github.com/aviatesk/JETLS.jl) (a Julia language
server):

> Before
<img width="924" height="160" alt="Screenshot 2026-07-15 at 16 05 40"
src="https://github.com/user-attachments/assets/b19fe9e3-50e3-44e4-9b4d-ac8bd438d3d6"
/>

> After
<img width="924" height="160" alt="Screenshot 2026-07-15 at 14 13 05"
src="https://github.com/user-attachments/assets/359c3382-118d-4fa6-aa91-6bd3661d3d28"
/>

---

Release Notes:

- Added support for rendering LSP 3.18 Markdown diagnostic messages from
language servers.

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Co-authored-by: Kirill Bulatov <mail4score@gmail.com>