dataconnect(change): Add 15 second grace before disconnecting a streaming connection with the backend

[dconeybe]

This PR adds a 15-second grace period before closing a Firebase Data Connect realtime streaming connection after the last subscriber unsubscribes, rather than closing it immediately. This helps avoid connection flapping and reduces backend load during rapid unsubscription/resubscription transitions, such as during activity or screen navigation.

Highlights

• Grace Period Implementation: Updated DataConnectBidiConnectStream to use a 15-second stop timeout (SharingStarted.WhileSubscribed) on the shared flow, delaying physical connection closure when the last subscriber unsubscribes.
• Unit Tests for Grace Period: Added a suite of unit tests in QuerySubscriptionImplUnitTest to verify that the connection remains open for exactly the grace period, that re-subscribing within the grace period reuses the connection, and that re-subscribing after the grace period establishes a new connection.
• Changelog Entry: Documented the change in the library's CHANGELOG.md.

Changelog

• CHANGELOG.md
  • Added an entry for the 15-second streaming connection grace period.
• DataConnectBidiConnectStream.kt
  • Configured SharingStarted.WhileSubscribed with a stopTimeoutMillis of 15,000 milliseconds (15 seconds) to delay connection closure when subscriptions are cancelled.
• QuerySubscriptionImplUnitTest.kt
  • Added unit tests checking the connection grace period duration, connection reuse during the grace period, new connection establishment after the grace period, and defined the CONNECTION_GRACE_PERIOD_MS constant.

[dconeybe]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a 15-second grace period before closing the realtime streaming connection after the last subscriber unsubscribes, replacing the previous behavior of closing it immediately. It also adds comprehensive unit tests to verify connection reuse within the grace period and connection recreation after it expires. The reviewer feedback suggests extracting the hardcoded 15-second duration (15,000 milliseconds) into a shared named constant to avoid magic numbers and ensure consistency between the implementation and the test suite.

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