feat(gateway): add readiness probe metrics and test-only store close

Emit Prometheus readiness metrics for database probes (healthy gauge and
outcome-labeled latency histogram) with coverage in health HTTP tests.
Restrict Store::close behind test support cfg to prevent accidental runtime
pool shutdown under live traffic.

Signed-off-by: Adrien Langou <alangou@nvidia.com>

Author: alangou

crates/openshell-server/Cargo.toml

@@ -91,6 +91,7 @@ nix = { workspace = true }
 
 [features]
 dev-settings = ["openshell-core/dev-settings"]
+test-support = []
 
 [dev-dependencies]
 hyper-rustls = { version = "0.27", default-features = false, features = ["native-tokio", "http1", "tls12", "logging", "ring", "webpki-tokio"] }