fix(plugin-mcp): bump @modelcontextprotocol/sdk from 1.25.2 to 1.27.1 (#15942)

# Overview

Bumps `@modelcontextprotocol/sdk` from `1.25.2` to `1.27.1` across the
monorepo.

## Key Changes

- Updated `@modelcontextprotocol/sdk` to `1.27.1` in
`packages/plugin-mcp`
- Updated `@modelcontextprotocol/sdk` to `^1.27.1` in
`test/package.json`
- Added a scoped pnpm override `mcp-handler>@modelcontextprotocol/sdk`
in root `package.json` to force `mcp-handler`'s hardcoded peer dep to
resolve to `1.27.1`

## Design Decisions

`mcp-handler@1.0.7` pins `@modelcontextprotocol/sdk@1.25.2` exactly as a
peer dependency, so a scoped override is needed to prevent both versions
from being installed side by side. The bump is backwards compatible —
the plugin only uses `McpServer` and `ResourceTemplate`, both of which
are unchanged.