fix(runtime): restore MCP fixture setup instructions - #7499

Summary

• Move setup_instructions ahead of child TOML tables in every pinned MCP registry fixture so it remains on the catalog root.
• Add a regression contract that parses every MCP fixture as McpCatalogEntry and requires non-empty root-level setup instructions.
• Synchronize the branch with the latest main.

Verification

• cargo test -p librefang-runtime --test registry_mcp_fixture_contract
• cargo clippy -p librefang-runtime --test registry_mcp_fixture_contract -- -D warnings
• cargo fmt --check
• git diff --check

Out of scope

• No runtime MCP parser or catalog schema changes.