Remove backend transport selection

[lucarlig]

Summary

• remove the Transport enum and BackendMCPGateway.transport field
• remove transport values from config fixtures, examples, and generated schema
• document Streamable HTTP as the unconditional upstream MCP transport

Why

The dataplane supports only Streamable HTTP. Keeping HTTP+SSE and STDIO choices
in the runtime model advertised transport paths that do not exist and made the
configuration surface misleading.

Impact

The generated UserConfig schema no longer declares or requires a backend
transport property. Control-plane config publishers should omit it; the
dataplane always constructs StreamableHttpClientTransport.

Validation

• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace --all-targets
• mdbook build docs/book
• mdbook test docs/book
• repository pre-commit hooks, including cargo deny, cargo nextest,
  workspace build, and cargo bench --no-run