feat: mcp-search tool for lazy loading mcp

[TheOutdoorProgrammer]

rebased #8771 onto the head of dev.

Closes #8625.

What does this PR do?

Adds a new experimental.mcp_lazy config option that prevents MCP tools from being loaded into context automatically. Instead, a new mcp_search tool is provided to discover and call MCP tools on-demand.

This reduces context usage for users with many MCP servers configured. When enabled:

• MCP tools are excluded from the tool list sent to the model

  • Instead, available MCP servers are injected into the system prompt in a compact format.

• The mcp_search tool allows listing, searching, describing, and calling MCP tools

• MCP servers show "Lazy" status in the /mcp dialog

The injected prompt:

<mcp_servers>
Available MCP servers: context7, firecrawl, playwright
Use mcp_search tool to discover and call tools from these servers.
</mcp_servers>

How did you verify your code works?

• Tested with experimental.mcp_lazy: true in config
• Verified MCP tools are not in context and mcp_search appears instead
• Tested all operations: list, search, describe, call
• Confirmed servers connect on-demand when calling tools

The tests were manual, there are no actual tests included here.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential Related PR Found:

PR #8771 - "feat: add mcp_lazy mode for on-demand MCP tool loading"
#8771

Why it's related: This is the original PR that the current PR #12520 is based on. According to the PR description, PR #12520 is a rebase of PR #8771 onto the head of dev. Both PRs address the same feature: implementing lazy loading for MCP tools with the mcp_search tool to discover and call tools on-demand instead of loading them automatically.

Since PR #12520 is explicitly stated to be a rebase of PR #8771, you should verify whether #8771 is still open or if it should be closed in favor of #12520.