* s3api: add optional request interceptor to circuit breaker
Add an optional Interceptor func(next http.HandlerFunc, action string) http.HandlerFunc
field on CircuitBreaker, applied at the very top of Limit() -- before upload
concurrency limiting and before the 'if !cb.Enabled' early return -- so it runs
for every route regardless of breaker state.
It is nil by default (no behavior change). An interceptor may reject a request
(write its own response and skip next) or wrap next to observe/shape it. This
provides a single per-request extension point at the existing per-route
chokepoint (cb.Limit already wraps nearly every S3 route), useful for tracing,
auditing, or request rate limiting without touching the route table.
* s3api: evaluate circuit breaker interceptor per request
Move the Interceptor nil-check into the returned handler so it is consulted
per request instead of captured at registration time. This:
- lets the interceptor be installed after registerRouter runs (handlers are
built during construction, before request-time dependencies exist), and
- avoids dereferencing a nil CircuitBreaker at registration time, which
panicked tests that register routes on a server with a nil cb
(e.g. TestRouting_STSWithQueryParams).
Adds a regression test for installing the interceptor after Limit() returns.
Chris Lu