Remove old setting to toggle the mermaid markdown tool

[mjbvz]

Instead you can now disable the tool itself, which is more consistent with the rest of our UX

Non-local session can use mermaid fenced code blocks

[Copilot]

Pull request overview

This PR removes the legacy mermaid-markdown.enabled setting that previously gated the Mermaid chat rendering tool, and instead restricts the contributed Mermaid tool to local chat sessions via a when clause—aligning with the direction of disabling tools directly rather than via separate feature settings.

Changes:

• Removed the mermaid-markdown.enabled configuration property from the extension manifest.
• Removed the associated localized description string.
• Updated the Mermaid language model tool contribution to be enabled only when chatSessionType == local.

Show a summary per file

File	Description
extensions/mermaid-markdown-features/package.nls.json	Removes the localization entry for the deleted mermaid-markdown.enabled setting description.
extensions/mermaid-markdown-features/package.json	Deletes the legacy setting and updates the Mermaid tool’s when clause to only apply to local chat sessions.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 1