Preserve custom-agent MCP server IDs during wire conversion

[krukow]

_{Generated via Copilot on behalf of @krukow}

Problem

Custom-agent :mcp-servers maps are serialized through recursive util/clj->wire. Keyword server IDs can therefore be transformed instead of treated as opaque identifiers, for example :srv-1 becoming srv1.

Expected behavior

Serialize nested custom-agent MCP servers with util/mcp-servers->wire, matching top-level session MCP server handling so IDs are preserved and normalized consistently.

Context

Discovered while reviewing #157. The behavior predates that PR and is tracked separately to keep the upstream-sync change focused.

Review thread: #157 (comment)