refactor: removes deprecated fields in filterapi (#1589)

**Description**

`metadataNamespace` and `modelNameHeaderKey` in the filterapi are no
longer used since v0.4, so this commit removes them.

Signed-off-by: Takeshi Yoneda <[email protected]>

Author: mathetake

cmd/aigw/testdata/translate_basic.out.yaml

@@ -217,8 +217,6 @@ stringData:
       schema:
         name: OpenAI
         version: v1
-    metadataNamespace: io.envoy.ai_gateway
-    modelNameHeaderKey: x-ai-eg-model
     models:
     - CreatedAt: "2025-05-23T00:00:00Z"
       Name: gpt-4o-mini

internal/controller/gateway.go

@@ -297,9 +297,6 @@ func (c *GatewayController) reconcileFilterConfigSecret(
 ) error {
 	// Precondition: aiGatewayRoutes is not empty as we early return if it is empty.
 	ec := &filterapi.Config{UUID: uuid}
-	// TODO: Drop this after v0.4.0.
-	ec.ModelNameHeaderKey = internalapi.ModelNameHeaderKeyDefault
-	ec.MetadataNamespace = aigv1a1.AIGatewayFilterMetadataNamespace
 	var err error
 	llmCosts := map[string]struct{}{}
 	for i := range aiGatewayRoutes {

internal/filterapi/filterconfig.go

@@ -28,17 +28,9 @@ var DefaultConfig = ``
 type Config struct {
 	// UUID is the unique identifier of the filter configuration assigned by the AI Gateway when the configuration is updated.
 	UUID string `json:"uuid,omitempty"`
-	// MetadataNamespace is the namespace of the dynamic metadata to be used by the filter.
-	// Deprecated.
-	// TODO: Drop this after v0.4.0.
-	MetadataNamespace string `json:"metadataNamespace"`
 	// LLMRequestCost configures the cost of each LLM-related request. Optional. If this is provided, the filter will populate
 	// the "calculated" cost in the filter metadata at the end of the response body processing.
 	LLMRequestCosts []LLMRequestCost `json:"llmRequestCosts,omitempty"`
-	// ModelNameHeaderKey is the header key to be populated with the model name by the filter.
-	// Deprecated.
-	// TODO: Drop this after v0.4.0.
-	ModelNameHeaderKey internalapi.ModelNameHeaderKey `json:"modelNameHeaderKey"`
 	// Backends is the list of backends that this listener can route to.
 	Backends []Backend `json:"backends,omitempty"`
 	// Models is the list of models that this route is aware of. Used to populate the "/models" endpoint in OpenAI-compatible APIs.

internal/filterapi/watcher_test.go

@@ -86,7 +86,6 @@ func testStartConfigWatcher(t *testing.T) {
 	cfg := `
 schema:
   name: OpenAI
-modelNameHeaderKey: x-ai-eg-model
 backends:
 - name: kserve
   weight: 1
@@ -117,7 +116,6 @@ backends:
 	cfg = `
 schema:
   name: OpenAI
-modelNameHeaderKey: x-ai-eg-model
 backends:
 - name: openai
   schema: