Add stateless/stateful mcp backend config

[npolshakova]

Description

Adds configuration for stateful/stateless mcp backends

Change Type

/kind feature

Changelog

Added configuration for stateful/stateless session routng for mcp backends. 

[Copilot]

Pull request overview

This PR adds configuration for stateful/stateless session routing for MCP backends. The new sessionRouting field allows users to specify whether requests from a session should be routed to the same backend replica (Stateful) or can be distributed across replicas (Stateless), with Stateful being the default behavior.

• Added SessionRouting type and field to MCPBackend struct in API
• Updated translation logic to convert sessionRouting configuration to statefulMode in backend output
• Updated CRD template to include the new enum field with validation

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
api/v1alpha1/agentgateway/agentgateway_backend_types.go	Defines the new SessionRouting type with Stateful/Stateless constants and adds the sessionRouting field to MCPBackend
pkg/kgateway/agentgatewaysyncer/backend/translate.go	Implements translation logic to convert sessionRouting API field to statefulMode backend field with STATEFUL as the default
install/helm/agentgateway-crds/templates/agentgateway.dev_agentgatewaybackends.yaml	Adds CRD validation for the new sessionRouting enum field
pkg/kgateway/agentgatewaysyncer/backend/testdata/backend/mcp-static-no-protocol.yaml	Updates test to verify Stateless session routing configuration
pkg/kgateway/agentgatewaysyncer/backend/testdata/backend/mcp-service-selector-sse.yaml	Updates test to verify Stateless session routing configuration with service selector

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[howardjohn]

LGTM just nitpick on the docs

[howardjohn]

This description is slightly inaccurate IMO. The primary aspect of Stateful mode is that an mcp-session will be created (via mcp-session-id). In order to make that behave properly we also then pin requests to a specific backend replica, but that is more of the secondary goal of the toggle -- the mcp session is user-facing while the pinning is internal.

[npolshakova]

Okay reworded here: b5416ee Let me know what you think!