PlacementException extends IllegalStateException, which neither BridgedApp
nor BridgeMcp's spawn catch blocks handled, so a maxLoad/quarantine/
all-exhausted refusal fell through to a blank 500 on REST and lost its
message on MCP. Catch it on both surfaces, ahead of the unrelated
IllegalArgumentException(unknown_profile) mapping, and return its message
structured: REST as {"error":"no_capacity","detail":...} with status 503,
MCP as an isError result prefixed "no capacity: ...".
Dai Ha