fix: remove artificial 500 message limit from sendEach

[ezhilnn]

Fixes #1103
#1103

Problem

sendEach makes an individual HTTP call per message, unlike the
deprecated sendAll which used a single HTTP batch request limited
by Google's batch API. The 500-message cap was copied from sendAll
where it was technically justified — but has no basis in sendEach.

Changes

• Removed the 500-message hard limit from sendEachOpAsync
• Updated Javadoc on sendEach and sendEachAsync methods to advise
  callers to chunk very large lists to avoid FCM server-side rate limiting

Testing

• Removed test asserting 500+ messages throws an exception
• Added test confirming 501+ messages are accepted

[gemini-code-assist]

Code Review

This pull request removes the 500-message limit from the sendEach and sendEachForMulticast methods in FirebaseMessaging, updating Javadocs and tests accordingly. A critical issue was identified in the new unit test where calling FirebaseMessaging.getInstance() without initializing FirebaseApp throws an IllegalStateException that is silently caught, resulting in a false positive test. It is recommended to use a mock client to properly verify the behavior.