Description

In admission/webhook/server.go, the admission webhook server has two issues:

1. In handleWebhookValidate, the validator is invoked with context.TODO() instead of req.Context(). As a result, if the API server disconnects or times out during validation, the validation cannot be cancelled, wasting CPU.
2. In parseRequest, the Content-Type header is checked with exact string equality (r.Header.Get("Content-Type") != "application/json"). Requests with standard media type parameters (such as application/json; charset=utf-8) are rejected with 400 Bad Request.

Steps To Reproduce

1. Send an AdmissionReview request with Content-Type: application/json; charset=utf-8 to /validate.
2. Notice the request fails with 400 Bad Request.
3. Observe that incoming request context cancellation does not propagate to validator.Validate().

Expected behavior

1. The webhook should accept application/json along with optional media type parameters such as charset=utf-8.
2. The webhook should pass req.Context() to validator.Validate() so cancellation propagates properly.

Actual Behavior

1. Requests with media type parameters are rejected with 400 Bad Request.
2. context.TODO() is passed to validator.Validate(), ignoring client cancellation or timeouts.