fix(v3rpc): make lease keep-alive stream processing synchronous to prevent goroutine leak

[HarshalPatel1972]

Description

Fixes #16942

This PR resolves a persistent gRPC lease keep-alive stream background goroutine leak that occurs during unexpected client disconnects, network partitions, or rapid context cancellations.

Technical Root Cause

Previously, LeaseKeepAlive decoupled the RPC lifecycle from the actual processing engine by spinning up ls.leaseKeepAlive(stream) inside an independent background goroutine (go func()), while the parent handler blocked on a separate select block listening to <-stream.Context().Done().

When a network drop or context cancellation unblocked the parent select, the parent handler exited immediately, signaling to gRPC that the stream was dead. However, the background processing thread remained permanently trapped and concurrently blocked on stream.Recv(). Because the stream context was already torn down, it could never return a clean io.EOF, resulting in silent goroutine aggregation and massive heap memory creep under volatile networking workloads.

Resolution Strategy

• Refactored LeaseKeepAlive to synchronously execute and return the loop handler: return ls.leaseKeepAlive(stream).
• Because stream.Recv() is natively context-aware, collapsing the async goroutine architecture binds the execution thread directly to the lifespans of the underlying network frames, allowing the runtime to drain out naturally when a cancel sequence fires.

Testing & Reproduction steps

• Implemented TestLeaseKeepAlive_GoroutineLeakOnDisconnect inside server/etcdserver/api/v3rpc/lease_test.go.
• The test mounts a mock stream handler, pushes load frames, triggers an unannounced context cancel step, and asserts via runtime.NumGoroutine() that the background reader exits instantly without leaving hanging execution contexts.

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