Consolidate sidecar injection into shared webhook logic

[cwiklik]

Description

This PR consolidates duplicate sidecar injection code by moving Agent CR injection from the kagenti operator into the kagenti-extensions webhook, where it shares common code with the existing MCPServer CR injection logic.

Problem Statement

Previously, sidecar injection logic existed in two places:

• kagenti operator: Directly injected sidecars into Agent CRs during reconciliation

• kagenti-extensions webhook: Injected sidecars into MCPServer CRs via admission webhook

This duplication created maintenance overhead, increased the risk of implementation drift, and made it harder to ensure consistent behavior across both resource types.

Solution

This PR implements a unified webhook-based approach:

• Created shared injection library (pkg/injection) containing common sidecar injection logic

• Refactored MCPServer webhook to use the shared library

• Added new Agent webhook that uses the same shared library

Both Agent and MCPServer CRs now receive identical sidecar injection through a consistent, maintainable codebase.

Flexible Injection Control

The webhook supports flexible injection control via namespace labels and annotations, similar to Istio's sidecar injection pattern. This provides fine-grained control over which resources receive sidecar injection. In deployments where the namespace is not specifically labeled to force sidecar injection, the Agent/MCPServer can opt-in via an annotation:

apiVersion: toolhive.stacklok.dev/v1alpha1
kind: MCPServer
metadata:
  name: force-injection-server
  namespace: other-namespace  # No label
  annotations:
    kagenti.dev/inject: "true"  # Explicit opt-in

If a namespace is labeled to force sidecar injection, the Agent/MCPServer can opt-out via an annotation:

apiVersion: v1
kind: Namespace
metadata:
  name: my-apps
  labels:
    kagenti-enabled: "true"  # All Agent/MCPServer CRs in this namespace get sidecars

apiVersion: toolhive.stacklok.dev/v1alpha1
kind: MCPServer
metadata:
  name: skip-injection-server
  namespace:   my-apps # All Agent/MCPServer CRs in this namespace get sidecars
  annotations:
    kagenti.dev/inject: "false"  # Explicit opt-out

Fixes #2

[Copilot]

Pull Request Overview

This PR consolidates duplicate sidecar injection logic by creating a shared mutation library used by both MCPServer and Agent webhooks. The changes eliminate code duplication between the kagenti operator and the webhook, implementing a unified webhook-based approach with Istio-style namespace injection controls.

Key Changes:

• Introduced shared pkg/injection library containing common pod mutation logic
• Refactored MCPServer webhook to use the new shared library
• Added new Agent webhook using the same shared mutation engine

Reviewed Changes

Copilot reviewed 64 out of 86 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
kagenti-webhook/internal/webhook/injector/pod_mutator.go	Core mutation engine implementing namespace-aware injection logic
kagenti-webhook/internal/webhook/injector/container_builder.go	Builds standardized sidecar containers (spiffe-helper, client-registration)
kagenti-webhook/internal/webhook/injector/volume_builder.go	Creates required volumes for sidecar containers
kagenti-webhook/internal/webhook/injector/namespace_checker.go	Checks namespace labels/annotations for injection enablement
kagenti-webhook/internal/webhook/v1alpha1/mcpserver_webhook.go	Refactored MCPServer webhook using shared mutator
kagenti-webhook/internal/webhook/v1alpha1/agent_webhook.go	New Agent webhook implementation using shared mutator
kagenti-webhook/cmd/main.go	Updated to instantiate shared mutator and register both webhooks
kagenti-webhook/go.mod	Added kagenti-operator dependency and k8s.io/utils
toolhive-webhook/*	Removed old toolhive-webhook files (replaced by kagenti-webhook)

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[pdettori]

/LGTM