swarm-controller: refuse a cap systemd would ignore; GET creates no bucket

systemd ignores a CPUQuota= or MemoryMax= it can't parse ("Invalid ...,
ignoring") and runs the unit without that limit. A swarm cap replaces
the hive's answer, so such a value left the agent uncapped while the
route, GET and the dashboard all said it was capped. argus on #4268 (R3)
found values the route took that systemd ignores: 12.345%, 400.123%,
0.001%, 17000000T, 99999999999999999999T and 99999999999999999999%.

The validators now accept a subset of what systemd parses, measured
against `systemd-analyze verify` (systemd 260):

- a percentage takes at most two decimals and is above zero, up to 100%
  for memory_max and 21474836.47% for cpu_quota (systemd keeps the quota
  in hundredths of a percent in a C int, not a u64)
- a size is one parse_size term: digits, an optional fraction, then
  nothing, B, K, M, G or T, case-sensitive. The whole part and the
  fraction digits fit u64, the whole part (plus one with a fraction)
  fits u64 once scaled, and the byte count is at least 1 and below
  2^64 - 1, as systemd checks them.

P and E, which systemd reads, stay refused because hive-c0re's validator
refuses them, and one value the hive refuses stops every cap update on
that hive. Values are stored as given: lower-case suffixes and a B
after a multiplier are refused rather than normalized, as systemd
refuses them.

The newline-injection values from the hive's test are in both bad lists
and go through the router in a_cap_with_a_second_line_is_a_400_and_the_caps_stay.

GET reads through a new agent_limits::open that returns None for a
missing bucket, answered as {}, instead of creating the bucket.

swarm-queue-client/src/agent_limits.rs

@@ -71,6 +71,30 @@ pub async fn open_or_create(
     .await
 }
 
+/// Open the caps bucket for reading, or `None` when nothing has created it
+/// yet, which reads the same as no caps.
+///
+/// **Controller-side**, for reads that must not create the bucket.
+///
+/// # Errors
+///
+/// [`crate::Error::OpenBucket`] for every failure other than the bucket not
+/// existing.
+#[cfg(feature = "kv")]
+pub async fn open(
+    client: &async_nats::Client,
+) -> Result<Option<async_nats::jetstream::kv::Store>, crate::Error> {
+    let js = async_nats::jetstream::new(client.clone());
+    match js.get_key_value(BUCKET).await {
+        Ok(store) => Ok(Some(store)),
+        Err(e) if crate::wanted::is_bucket_not_found(&e) => Ok(None),
+        Err(source) => Err(crate::Error::OpenBucket {
+            bucket: BUCKET.to_owned(),
+            source,
+        }),
+    }
+}
+
 /// What reading one agent's key found.
 #[cfg(feature = "agent-limits")]
 #[derive(Debug)]