feat: passthrough upstream response headers to clients

CPA previously stripped ALL response headers from upstream AI provider
APIs, preventing clients from seeing rate-limit info, request IDs,
server-timing and other useful headers.

Changes:
- Add Headers field to Response and StreamResult structs
- Add FilterUpstreamHeaders helper (hop-by-hop + security denylist)
- Add WriteUpstreamHeaders helper (respects CPA-set headers)
- ExecuteWithAuthManager/ExecuteCountWithAuthManager now return headers
- ExecuteStreamWithAuthManager returns headers from initial connection
- All 11 provider executors populate Response.Headers
- All handler call sites write filtered upstream headers before response

Filtered headers (not forwarded):
- RFC 7230 hop-by-hop: Connection, Transfer-Encoding, Keep-Alive, etc.
- Security: Set-Cookie
- CPA-managed: Content-Length, Content-Encoding

sdk/api/handlers/openai/openai_responses_compact_test.go

@@ -31,7 +31,7 @@ func (e *compactCaptureExecutor) Execute(ctx context.Context, auth *coreauth.Aut
 	return coreexecutor.Response{Payload: []byte(`{"ok":true}`)}, nil
 }
 
-func (e *compactCaptureExecutor) ExecuteStream(context.Context, *coreauth.Auth, coreexecutor.Request, coreexecutor.Options) (<-chan coreexecutor.StreamChunk, error) {
+func (e *compactCaptureExecutor) ExecuteStream(context.Context, *coreauth.Auth, coreexecutor.Request, coreexecutor.Options) (*coreexecutor.StreamResult, error) {
 	return nil, errors.New("not implemented")
 }