Function append_pinned_mcp_section 

pub fn append_pinned_mcp_section(
    deferred_section: &mut String,
    pinned_section: &str,
)

Append a pre-rendered pinned-MCP-resources section onto the system-prompt MCP accumulator (deferred_section).

This MUST be called after the deferred_loading branch, which reassigns deferred_section with = (via build_deferred_tools_section_filtered) and would otherwise clobber any earlier-pushed pinned content. Centralizing the append keeps both run() and process_message() consistent and pins the ordering invariant in one testable place. No-op for an empty section.