fix(multiplayer): bind signed workload tokens to allocation_id only

The just-landed signed workload token embedded (allocation_id, match_id,
server_id) as claims. That doesn't actually work for its intended delivery
channel: the token is meant to be requested as a GameServerAllocation
annotation in the SAME request that asks Agones to pick a server, so at mint
time the allocator knows allocation_id (it generates it) but not yet which
server_id Agones will return -- server_id only exists in Agones's response,
after the annotation request has already been sent. Embedding it was simply
not possible for the real caller this was built for; only the (allocator ->
signed_token) unit tests and hand-constructed integration tests happened to
supply it directly, masking the gap.

Fixes it by having the token bind only allocation_id (the one identifier
actually known at mint time) plus expiry. match_id/server_id are resolved at
verify time from the durable allocations table via the new
store.AllocationBindingByAllocationID, keyed by allocation_id -- which the
allocator already records immediately after Agones responds. This is
strictly stronger, not just a workaround: a caller can no longer claim any
match/server pairing at all, even one that happens to be internally
consistent -- the binding returned is entirely durable-record-derived.

Verified: server/workload's unit tests updated for the new two-field claim
shape; server/api's Postgres integration suite gains
TestWorkloadVerifierFromSignedTokenNeverTrustsCallerSuppliedBinding (two
distinct real allocations each resolve to their own, and only their own,
match/server pairing) replacing the now-inapplicable mismatched-triple test.
Full `go build ./... && go vet ./... && gofmt -l . && go test ./... -race`
and `go test -tags integration ./... -race` both clean; the api integration
suite re-run 3x clean against a live postgres:17-alpine container.

server/workload/signed_token.go

@@ -18,22 +18,31 @@ import (
 // cluster to validate against and so cannot be built or tested here.
 //
 // This sidesteps that requirement entirely: the control plane signs its own
-// short-lived token over (allocation_id, match_id, server_id, expiry) with a
-// secret only it holds, exactly the way domain.SessionStore already mints
-// player session tokens elsewhere in this codebase. It needs no Kubernetes
-// trust boundary to verify -- HMAC signature plus expiry is self-contained.
+// short-lived token over (allocation_id, expiry) with a secret only it
+// holds, exactly the way domain.SessionStore already mints player session
+// tokens elsewhere in this codebase. It needs no Kubernetes trust boundary
+// to verify -- HMAC signature plus expiry is self-contained.
+//
+// The token deliberately binds ONLY allocation_id, not match_id/server_id
+// too: it is meant to be requested as a GameServerAllocation annotation
+// (see agones/allocation.go) in the SAME request that asks Agones to pick a
+// server for this allocation -- so at mint time, the allocator knows
+// allocation_id (it generates it) but not yet which server_id Agones will
+// return. match_id and server_id are instead resolved durably at verify
+// time from the allocations table, which the allocator records immediately
+// after Agones responds (see store.AllocationBindingByAllocationID) -- so a
+// token can never claim a match/server pairing that isn't what was actually,
+// durably allocated.
 //
 // The delivery channel is what makes this safe despite not proving pod
-// identity the way a Kubernetes-issued token would: the token is meant to be
-// handed to the allocated GameServer via the same Agones GameServerAllocation
-// annotation channel allocation.go already uses for match-id/allocation-id
-// (see agones/allocation.go), which only the actually-allocated pod's local
-// SDK sidecar can read. A caller who can present this token has already
-// proven, via that channel, that it is the pod Agones allocated.
+// identity the way a Kubernetes-issued token would: the token reaches the
+// allocated GameServer via the same annotation channel allocation.go
+// already uses for match-id/allocation-id, which only the actually-
+// allocated pod's local SDK sidecar can read. A caller who can present this
+// token has already proven, via that channel, that it is the pod Agones
+// allocated.
 type SignedWorkloadToken struct {
 	AllocationID string    `json:"a"`
-	MatchID      string    `json:"m"`
-	ServerID     string    `json:"s"`
 	ExpiresAt    time.Time `json:"e"`
 }
 
@@ -46,15 +55,15 @@ var (
 )
 
 // IssueSignedWorkloadToken produces a compact "payload.signature" token
-// binding the three identifiers the API layer actually checks (see
-// api.Service's WorkloadVerify call site: it only compares ServerID and
-// MatchID on the returned domain.WorkloadBinding). now must be non-zero and
-// ttl must be positive so a token is never silently issued already-expired.
-func IssueSignedWorkloadToken(secret []byte, allocationID, matchID, serverID string, now time.Time, ttl time.Duration) (string, error) {
+// binding allocation_id, the one identifier known at mint time (see the
+// type doc above for why match_id/server_id aren't embedded). now must be
+// non-zero and ttl must be positive so a token is never silently issued
+// already-expired.
+func IssueSignedWorkloadToken(secret []byte, allocationID string, now time.Time, ttl time.Duration) (string, error) {
 	if len(secret) == 0 {
 		return "", ErrEmptyWorkloadSecret
 	}
-	if allocationID == "" || matchID == "" || serverID == "" {
+	if allocationID == "" {
 		return "", ErrTokenClaims
 	}
 	if now.IsZero() || ttl <= 0 {
@@ -62,8 +71,6 @@ func IssueSignedWorkloadToken(secret []byte, allocationID, matchID, serverID str
 	}
 	claims := SignedWorkloadToken{
 		AllocationID: allocationID,
-		MatchID:      matchID,
-		ServerID:     serverID,
 		ExpiresAt:    now.Add(ttl).UTC(),
 	}
 	payload, err := json.Marshal(claims)
@@ -113,7 +120,7 @@ func ParseSignedWorkloadToken(secret []byte, token string, now time.Time) (Signe
 	if err := json.Unmarshal(payload, &claims); err != nil {
 		return SignedWorkloadToken{}, ErrMalformedToken
 	}
-	if claims.AllocationID == "" || claims.MatchID == "" || claims.ServerID == "" || claims.ExpiresAt.IsZero() {
+	if claims.AllocationID == "" || claims.ExpiresAt.IsZero() {
 		return SignedWorkloadToken{}, ErrTokenClaims
 	}
 	if now.IsZero() {