mcp/mcp_router: add initial multiplexing and aggregation supports

[botengyao]

MCP Request/Response Handling

Operation	Direction	Handling
initialize	Fanout	Aggregate session IDs into composite
tools/list	Fanout	Aggregate tools, prefix names
tools/call	Targeted	Strip prefix, route to specific backend
ping	Fanout	Return 200 with empty JSON response
notification/initialized	Fanout	Return 202

Key Decisions

• Handle the streamable HTTP for MCP POST request.
• This PR only handles the application/json content, event/stream response should also be supported but not this PR.
• Body rewrite for tools/call: Use string search to replace prefixed tool name.
  -- The mcp decoder could provide a cursor to be more efficient and more robust
• Content-Length handling: Set explicit length after body modification
• Response aggregation: Full JSON parsing required for tools/list (multiple tool names to prefix)
• mcp-session-id is the combined one from multiple backends, and there should be a limit.
• Capability is gateway default now and should be controlled by the gateway.
• Should combine with the mcp_filter. And the request flow is mcp_fitler -> other_filters (attributes aware) -> mcp_router -> backends.

#39174

static_resources:
  listeners:
    - name: mcp_listener
      address:
        socket_address:
          address: 0.0.0.0
          port_value: 10001
      filter_chains:
        - filters:
            - name: envoy.filters.network.http_connection_manager
              typed_config:
                "@type": type.googleapis.com/envoy.extensions.filters.network.http_connection_manager.v3.HttpConnectionManager
                stat_prefix: ingress_http
                route_config:
                  name: local_route
                  virtual_hosts:
                    - name: mcp_gateway
                      domains: ["gateway.com"]
                      routes:
                        - match:
                            path: "/mcp"
                          route:
                            cluster: mcp_service
                http_filters:
                  - name: envoy.filters.http.mcp
                    typed_config:
                      "@type": type.googleapis.com/envoy.extensions.filters.http.mcp.v3.Mcp
                      traffic_mode: REJECT_NO_MCP
                      clear_route_cache: true
                  - name: envoy.filters.http.mcp_router
                    typed_config:
                      "@type": type.googleapis.com/envoy.extensions.filters.http.mcp_router.v3.McpRouter
                      servers:
                        - name: echo_one
                          mcp_cluster:
                            cluster: mcp_service
                            path: "/mcp"
                        - name: echo_two
                          mcp_cluster:
                            cluster: mcp_service_2
                            path: "/mcp"
  clusters:
    - name: mcp_service
      type: STATIC
      lb_policy: ROUND_ROBIN
      load_assignment:
        cluster_name: mcp_service
        endpoints:
        - lb_endpoints:
          - endpoint:
              address:
                socket_address:
                  address: 127.0.0.1
                  port_value: 9000

    - name: mcp_service_2
      type: STATIC
      lb_policy: ROUND_ROBIN
      load_assignment:
        cluster_name: mcp_service
        endpoints:
        - lb_endpoints:
          - endpoint:
              address:
                socket_address:
                  address: 127.0.0.1
                  port_value: 9001

Commit Message:
Additional Description:
Risk Level:
Testing:
Docs Changes:
Release Notes:
Platform Specific Features:

[repokitteh-read-only]

As a reminder, PRs marked as draft will not be automatically assigned reviewers,
or be handled by maintainer-oncall triage.

Please mark your PR as ready when you want it to be reviewed!

🐱

Caused by: #42456 was opened by botengyao.

see: more, trace.

[repokitteh-read-only]

CC @envoyproxy/api-shepherds: Your approval is needed for changes made to (api/envoy/|docs/root/api-docs/).
envoyproxy/api-shepherds assignee is @mattklein123
CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

🐱

Caused by: #42456 was opened by botengyao.

see: more, trace.

[yanavlasov]

/wait-any

[botengyao]

Thanks for the review!

[yanavlasov]

/wait

[yanavlasov]

LGTM. Let's just split config into a separate file and will then submit.

/wait

[yanavlasov]

Bypassing API review as we are changing it very repidly and it will be likely to be quite different in its stable form. Will review API separately once we have stabilized it.