### What problem does this PR solve?
- Migrated MCP server detail and export (download) API from Python to
Go.
- Registered route: `GET /api/v1/mcp/servers/:mcp_id` (supporting
`?mode=download` query parameter).
## What
#15240
implementation for PUT /api/v1/mcp/servers/:mcp_id
## Changes
- Adds the Go implementation for `PUT /api/v1/mcp/servers/:mcp_id`.
- Wires MCP service and handler into the Go server/router for the update
route.
- Preserves Python-style behavior for ownership checks, partial update
fields, MCP type/name/URL validation, `headers`/`variables`
normalization, and tool metadata scrubbing.