transport/http: depends on equality of pointer to zero-sized objects

In `transport/http/identity_test.go`, `TestIdentity` compares the result from `resolver.GetIdentity` with `&auth.AnonymousIdentity{}`. That test is fragile, as the behavior of comparing pointers to zero-sized objects (which `auth.AnonymousIdentity` is) is not guaranteed (https://go.dev/ref/spec#Comparison_operators, bullet point 6).

This test, and presumably other clients of `auth.IdentityResolver`s, should use a type assertion instead of interface equality.

Replace
```
if expected != actual {
```
with
```
if _, ok := actual.(*auth.AnonymousIdentity); !ok {
```
(Or a sentinel *value* should be defined in `auth` instead of a sentinel *type*.)

We believe this test will fail in the upcoming go1.25 (~August 2025) because of optimizations that change the behavior of equality on pointer-to-zero-sized allocations.

Start in transport/http/identity_test.go at TestIdentity and trace the resolver.GetIdentity result through the auth.IdentityResolver interface. Run the test before and after the change; done means the test checks the returned identity type without relying on equality between pointers to zero-sized objects, and any other identified clients use the same safe approach.

Written by the indexing model from the issue text.