extproc crashes on 403 forbidden error

[yuzisun]

Description:
AI Gateway extproc crashes when getting 403 forbidden error, this is a regression from the recent PR f07a7ff

time=2025-02-23T18:27:15.814Z level=DEBUG msg="response headers processing" response_headers="headers:{key:\":status\" raw_value:\"403\"}"
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0xe81a2f]

goroutine 162 [running]:
github.com/envoyproxy/ai-gateway/internal/extproc.(*Server).processMsg(0xc0000c0048, {0x1357900, 0xc0003e6ae0}, {0x0, 0x0}, 0x15?)
	/Users/dsun20/go/src/github.com/envoyproxy/ai-gateway/internal/extproc/server.go:211 +0x44f
github.com/envoyproxy/ai-gateway/internal/extproc.(*Server).Process(0xc0000c0048, {0x135edf8, 0xc0000c61d0})
	/Users/dsun20/go/src/github.com/envoyproxy/ai-gateway/internal/extproc/server.go:169 +0x2a5
github.com/envoyproxy/go-control-plane/envoy/service/ext_proc/v3._ExternalProcessor_Process_Handler({0x10e2280?, 0xc0000c0048}, {0x135be50, 0xc0000001c0})
	/Users/dsun20/go/pkg/mod/github.com/envoyproxy/go-control-plane/[email protected]/service/ext_proc/v3/external_processor_grpc.pb.go:106 +0xd8
google.golang.org/grpc.(*Server).processStreamingRPC(0xc000396e00, {0x1357900, 0xc0003e6a20}, 0xc0002e49c0, 0xc0003b19e0, 0x1c6d380, 0x0)
	/Users/dsun20/go/pkg/mod/google.golang.org/[email protected]/server.go:1690 +0x1252
google.golang.org/grpc.(*Server).handleStream(0xc000396e00, {0x1357d08, 0xc000290820}, 0xc0002e49c0)
	/Users/dsun20/go/pkg/mod/google.golang.org/[email protected]/server.go:1814 +0xb47
google.golang.org/grpc.(*Server).serveStreams.func2.1()
	/Users/dsun20/go/pkg/mod/google.golang.org/[email protected]/server.go:1030 +0x7f
created by google.golang.org/grpc.(*Server).serveStreams.func2 in goroutine 161
	/Users/dsun20/go/pkg/mod/google.golang.org/[email protected]/server.go:1041 +0x11d

Repro steps:
The crash can be replicated if the ext authz filter returns 403, e.g user is not permissioned to access the model

Logs:

[2025-02-20T14:59:14.595Z] "POST /v1/chat/completions HTTP/1.1" 403 UAEX 129 0 7 "10.244.87.192" "curl/8.7.1" "8b08965a-0cde-4953-87f9-56918343b549" "llama-3-1-70b-instruct-vllm.example.com" "-" "envoy-gateway-eg-fef97bb3-76pvt"

[yuzisun]

cc @nacx @mathetake

[mathetake]

lol i referenced the issue number 403 instead of 419