fix topic subscription when @ and + are encoded

[tshedor]

FirebaseMessaging.subscribeToTopic() / unsubscribeFromTopic() produced a 400 on any topic containing %, even though the documented grammar is [a-zA-Z0-9-_.~%]{1,900}.

Introduced in firebase-messaging 25.1.0 (BoM 34.15.0, 16 Jun 2026) when FID registration landed (#8087) and TopicSubscriptionClient started concatenating the topic unescaped into https://fcmregistrations.googleapis.com/v1/projects/{id}/registrations/{fid}/topicSubscriptions/{topic}:{op}.

The server percent-decodes the path before validating, so fields like %2B or %40 become + / @ and the request is rejected.

This is still present in 25.1.1 / 25.1.2. Pre-25.1.0 (GmsRpc, last good BoM 34.14.0 / messaging 25.0.2) sent the topic as a gcm.topic value, so % survived.

This PR percent-escapes the topic path segment (% → %25) on both subscribe and unsubscribe so the wire form matches the documented name. Same class of bug as firebase/quickstart-ios#242, fixed by firebase-ios-sdk#220.

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