fix(features): client implies openapi, per the spec

AGENTS.md, lib.rs, the README, and adapters/mod.rs docs all state
openapi is the shared spec model implied by both feature sides, and
the README's client row claims from_openapi under client — but the
feature graph only had server -> openapi. A lean client-only build
compiled yet silently lacked from_openapi (gated all(client,
openapi)).

Verification: cargo test (default + all-features), clippy both
feature sets -D warnings, fmt, publish --dry-run — all green.

Cargo.toml

@@ -25,7 +25,7 @@ server = ["dep:axum", "dep:hyper", "dep:hyper-util", "dep:uuid", "openapi", "htt
 # Consumer side: the shared outbound client host and the HTTP-backed
 # import adapters (`from_jsonschema`, `from_openapi` with `openapi`,
 # `from_mcp` with `mcp`).
-client = ["dep:arc-swap", "dep:httpdate", "dep:percent-encoding", "dep:reqwest", "dep:reqwest-middleware", "dep:reqwest-retry", "dep:url"]
+client = ["dep:arc-swap", "dep:httpdate", "dep:percent-encoding", "dep:reqwest", "dep:reqwest-middleware", "dep:reqwest-retry", "dep:url", "openapi"]
 # Shared OpenAPI document model: `from_openapi` parsing (client side)
 # and the `to_openapi` gateway projection (server side).
 openapi = ["dep:yaml_serde"]