Fix vLLM ZMQ topic filter

[avinxshKD]

What type of PR is this?

/kind bug

What this PR does / why we need it:

The vLLM ZMQ subscriber already subscribes to the topic configured by VLLM_ZMQ_TOPIC_FILTER, but the message filtering path still compared incoming topics against the hardcoded kv-events topic.

That made non-default topic filters silently drop valid messages after they were received from ZMQ.

This PR makes the vLLM subscriber use the configured topic for the application-level topic check, matching the existing SGLang subscriber behavior.

Which issue(s) this PR fixes:
Fixes #1527

Bug evidence (required for bug-related PRs):

Set VLLM_ZMQ_TOPIC_FILTER=test-topic and publish a normal 3-part vLLM ZMQ message with topic test-topic.

Before this change:

• the SUB socket subscribes to test-topic
• the frame is received from ZMQ
• the application-level check drops it because the code compares the topic with hardcoded kv-events
• the event is never processed

The broken path was:

topic, payload = self._extract_message_data(parts)
if not topic or topic != "kv-events":
    continue

After this change, the same application-level check uses self.config.zmq_topic_filter, so the topic used by the SUB socket and the topic accepted by the subscriber stay consistent.

A regression test covers the non-default topic case and verifies that a message on the configured topic is processed while a different topic is ignored.

Special notes for your reviewer:
The change intentionally mirrors the existing SGLang ZMQ subscriber topic-filter handling.

Does this PR introduce a user-facing change?:

Fixed vLLM ZMQ event subscriber topic filtering to honor VLLM_ZMQ_TOPIC_FILTER during message processing.

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[avinxshKD]

Hey @FAUST-BENCHOU @acsoto PTAL. Kept this intentionally small, mirrors the SGLang topic check and added a narrow regression for non-default vLLM topics.

cc @LiZhenCheng9527 @YaoZengzeng

[Copilot]

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[acsoto]

/lgtm

[LiZhenCheng9527]

/approve

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