Refuse a key-value write too large for its bucket before it is put (issue 774)

mesh-grants' sealed grants and the controller's merge record each met "message size exceeds maximum" (JetStream
10054) only after the put, and a writer that retries looped on a value that can never land; an update over the
limit was even answered as a revision mismatch, which every compare-and-set writer retries.

Quick fix: cap the two values in their modules. Thorough fix, chosen because the kind was met twice and every
module writes through the SDK: the package kvlimit checks a value against its bucket's largest message (read from
its stream and kept) before the put, naming the bucket, the key, the size and the limit, and turns the bus's own
refusal into the same *TooLarge (errors.Is kvlimit.ErrTooLarge), so no writer retries it. The SDK's own writers
use it: process.State (Put, Create, CreateTTL, Update), process.AwaitedBucket, the runtime state of a bundle
(stdio KeptState, against the module state's 256 KiB) and mesh-hand-compose. A value that can outgrow its bucket
is split over keys or kept in the object store with its name in the key.

go/state_test.go

@@ -122,6 +122,21 @@ func TestABundleKeepsAndWatchesStateThroughTheRuntime(t *testing.T) {
 	if r := next(); r["id"] != float64(2) || r["result"].(map[string]any)["content"].([]any)[0].(map[string]any)["text"] != "9" {
 		t.Fatalf("the put answered %v", r)
 	}
+	// Issue 774 (sweep item 5): a value over a module state's largest is refused before the runtime is asked, naming the
+	// key, its size and the limit; one the runtime's bus refuses anyway comes back as the same refusal.
+	send(map[string]any{"jsonrpc": "2.0", "id": 30, "method": "tools/call", "params": map[string]any{"name": "remember",
+		"arguments": map[string]any{"key": "all.big", "value": strings.Repeat("x", 300*1024)}}})
+	if r := next(); r["id"] != float64(30) || !strings.Contains(fmt.Sprint(r), "all.big in the state servers is 307202 bytes, over the 262144") {
+		t.Fatalf("a value over the limit was not refused before the runtime was asked: %v", r)
+	}
+	send(map[string]any{"jsonrpc": "2.0", "id": 31, "method": "tools/call", "params": map[string]any{"name": "remember",
+		"arguments": map[string]any{"key": "all.near", "value": "x"}}})
+	near := next()
+	send(map[string]any{"jsonrpc": "2.0", "id": near["id"], "error": map[string]any{"code": -32000,
+		"message": "nats: API error: code=400 err_code=10054 description=message size exceeds maximum allowed"}})
+	if r := next(); r["id"] != float64(31) || !strings.Contains(fmt.Sprint(r), "written again it is refused again") {
+		t.Fatalf("the bus's refusal was not named as too large: %v", r)
+	}
 	// PutTTL and CreateTTL ask the runtime's put and create with the key's time to live in whole seconds; a create the
 	// runtime refuses as changed (-32010) is ErrChanged.
 	for i, create := range []bool{false, true} {