agents: read every MemoryMax= size systemd enforces

`parse_bytes` knew only K/M/G/T, in either case. A size with the bare
`B` unit (`1048576B`), which `validate_memory_max` stores and systemd
enforces, became `infinity` in the agent's cap file: the cgroup was
capped while the agent was told there was no cap. It now takes exactly
the single-term forms systemd's `parse_size` does: no suffix, `B`, or
K/M/G/T/P/E, case-sensitive. Lowercase units and `4GB`, which systemd
refuses and so leaves the unit uncapped, read as `infinity` too.

A test runs every suffix form the validator accepts through it and
checks the stored value parses to the right byte count.

Part of #5081 (slice S0-2).

hive-c0re/src/lifecycle/host_config.rs

@@ -947,6 +947,8 @@ mod tests {
     fn a_cap_is_read_the_way_systemd_reads_memory_max() {
         for (memory_max, cap) in [
             ("4G", "4294967296"),
+            ("1048576B", "1048576"),
+            ("3P", "3377699720527872"),
             ("infinity", "infinity"),
             ("50%", "8192004096"),
             ("33.33%", "5460787200"),
@@ -954,6 +956,8 @@ mod tests {
             ("100%", "16384012288"),
             // systemd ignores these and the unit runs uncapped.
             ("0", "infinity"),
+            ("4g", "infinity"),
+            ("4GB", "infinity"),
             ("0%", "infinity"),
             ("150%", "infinity"),
             ("12.345%", "infinity"),