Closes #31351

# Diagnostics Mention in New Threads

## Overview

Adds the `@diagnostics` mention to the new Agent Panel threads so users
can inject current LSP diagnostics (errors by default) without switching
to a text thread. The diagnostics mention is fully integrated into ACP
mention parsing, the context picker, and the message-editor pipeline so
it round-trips cleanly and shows up in the standard `@` menu.

## Context

- **Request:** bring `/diagnostics` parity to the “New Thread” assistant
experience.
- **Scope:** diagnostics only; `/terminal` mention would be implemented
in a separate PR.
- **Docs:** updated Agent Panel docs + changelog.

## Implementation Details

1. **Mention plumbing**
- Added `MentionUri::Diagnostics` to `acp_thread`, including parsing
(`zed:///agent/diagnostics?include_warnings=true`) and icon/name
metadata.
- Tests ensure diagnostics links round-trip via Markdown mention
serialization.

2. **Context picker / completion**
- New `ContextPickerMode::Diagnostics` exposes an `@diagnostics` entry
in the mention menu.
- Completions turn `@diagnostics` into a fully fledged mention, reusing
the existing confirmation pipeline.

3. **Message editor + thread serialization**
- Resolving the mention calls the existing diagnostics collector from
`assistant_slash_commands`, embedding the tool output inline with other
context blocks (`<diagnostics>…</diagnostics>`).
- Thread-link handling ignores diagnostics backlinks so clicking them
doesn’t try to reopen nonexistent resources.
   

# How it looks
<img width="800" height="480" alt="image"
src="https://cf5gpe8lxo.ufs.sh/f/EmJ5Xl877qJO1mzC9Zrn8AmJZHeShC4RoUwvTMlF2tfPzj06"
/>

Release Notes:

- Allow mentioning diagnostics in the agent panel via `@diagnostics`

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Co-authored-by: Bennet Bo Fenner <bennet@zed.dev>