fix(pubsub/v2): skip stream keep-alive when using binary emulator

[shairoth12]

Problem

Fixes #14358

pubsub/v2 v2.5.0 introduced dead-stream detection (streamKeepAliveHandler) that sends periodic keep-alive pings and tears down the stream if the server doesn't respond. The binary PubSub emulator (gcloud beta emulators pubsub start) silently discards these pings, causing repeated false stream teardowns and 15–45s message delivery delays.

Solution

Skip streamKeepAliveHandler when PUBSUB_EMULATOR_HOST is set — the same env var already used to configure insecure credentials for the emulator. Tickers are still created unconditionally to avoid nil-guard changes throughout the codebase; they simply tick into the void.

Changes

• iterator.go: gate go it.streamKeepAliveHandler() behind os.Getenv("PUBSUB_EMULATOR_HOST") == ""
• iterator_test.go: add TestStreamingPullKeepAlive_Disabled — mirrors the existing reconnect test with aggressive intervals and protocolVersion=0, but asserts openCount == 1 (no reconnects) when the env var is set

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[gemini-code-assist]

Code Review

This pull request modifies the PubSub iterator to skip dead-stream detection when the PUBSUB_EMULATOR_HOST environment variable is set, preventing unnecessary stream teardowns when using the emulator. A critical deadlock issue was identified in the implementation: the WaitGroup is incremented unconditionally, but the goroutine responsible for decrementing it is skipped when the emulator is active, which will cause the iterator's stop method to hang indefinitely.

removing review in favor of emulator change