Add configurable heartbeat interval for unload events

[fengfeng-zi]

Summary

• add a public heartbeat_interval launch/mount option for the session heartbeat stream
• use that configured interval in the unload heartbeat route instead of the hidden e2e env shortcut
• update the unload demo and add regression coverage for launch/mount parity

Closes #13347

Testing

• python -m pytest test/test_server.py::TestServer::test_server_launch_sets_heartbeat_interval test/test_server.py::test_server_launch_args_match_blocks_launch test/test_routes.py::TestRoutes::test_mount_gradio_app_sets_heartbeat_interval test/test_routes.py::test_mount_gradio_app_args_match_launch_args -q

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Note

Medium Risk
Changes the server-side session heartbeat timing that drives unload behavior; misconfiguration (too low/high) could increase server load or delay session cleanup/unload triggers.

Overview
Adds a public heartbeat_interval option (default 15.0s) to Blocks.launch(), gr.mount_gradio_app(), and Server.launch(), storing it on the Blocks instance and validating it must be > 0.

Updates the /heartbeat/{session_hash} route to use the configured interval instead of an internal env-var-based override, making unload event timing tunable.

Updates the unload_event_test demo to launch with a shorter interval and adds regression tests ensuring heartbeat_interval is correctly plumbed through mount_gradio_app and Server.launch.

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