Skip Envoy body processing for prefix-free 2026-07-28 routes

[david-martin]

From future work captured in

mcp-gateway/docs/design/router-2026-07-28/router-2026-07-28-design.md

Line 219 in caf6847

- **Body phase skip.** When no prefix is configured on any MCPServerRegistration, the ext_proc could be configured with `request_body_mode: NONE` for `2026-07-28` routes, eliminating the body phase entirely at the Envoy level.

For 2026-07-28 routes, skip Envoy’s ext_proc request body phase when no MCPServerRegistration uses a tool prefix. Configure request_body_mode: NONE when prefix stripping is unnecessary.

This removes buffering and parsing from the tool-call hot path, reducing CPU, memory use, and latency while enabling Envoy-level streaming.

Current performance testing shows that 2025 and 2026 tool-call results are nearly identical. This post-GA 1.1 optimization should make the expected 2026 performance advantage measurable.

[RichaShiny]

I’d like to work on this. I’ll first map how prefix configuration reaches the EnvoyFilter, then implement the smallest change with focused controller tests verifying request_body_mode: NONE for prefix-free 2026 routes while preserving buffered processing where rewriting is required.