pageserver: return 503 during tenant shutdown (#9635)

## Problem

Tenant operations may return `409 Conflict` if the tenant is shutting
down. This status code is not retried by the control plane, causing
user-facing errors during pageserver restarts. Operations should instead
return `503 Service Unavailable`, which may be retried for idempotent
operations.

## Summary of changes

Convert
`GetActiveTenantError::WillNotBecomeActive(TenantState::Stopping)` to
`ApiError::ShuttingDown` rather than `ApiError::Conflict`. This error is
returned by `Tenant::wait_to_become_active` in most (all?)
tenant/timeline-related HTTP routes.

pageserver/src/http/routes.rs

@@ -37,6 +37,7 @@ use pageserver_api::models::TenantShardLocation;
 use pageserver_api::models::TenantShardSplitRequest;
 use pageserver_api::models::TenantShardSplitResponse;
 use pageserver_api::models::TenantSorting;
+use pageserver_api::models::TenantState;
 use pageserver_api::models::TimelineArchivalConfigRequest;
 use pageserver_api::models::TimelineCreateRequestMode;
 use pageserver_api::models::TimelinesInfoAndOffloaded;
@@ -295,6 +296,9 @@ impl From<GetActiveTenantError> for ApiError {
             GetActiveTenantError::Broken(reason) => {
                 ApiError::InternalServerError(anyhow!("tenant is broken: {}", reason))
             }
+            GetActiveTenantError::WillNotBecomeActive(TenantState::Stopping { .. }) => {
+                ApiError::ShuttingDown
+            }
             GetActiveTenantError::WillNotBecomeActive(_) => ApiError::Conflict(format!("{}", e)),
             GetActiveTenantError::Cancelled => ApiError::ShuttingDown,
             GetActiveTenantError::NotFound(gte) => gte.into(),