chore: Use (*OpenAIRouter)(nil) for interface compliance check

[cryo-zd]

What type of PR is this?

chore: Use (*OpenAIRouter)(nil) for interface compliance check

What this PR does / why we need it:
This avoids unnecessary allocation at package init and makes the intent clearer, following common Go idioms.

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[github-actions]

👥 vLLM Semantic Team Notification

The following members have been identified for the changed files in this PR and have been automatically assigned:

📁 src

Owners: @rootfs, @Xunzhuo, @wangchen615
Files changed:

• src/semantic-router/pkg/extproc/router.go

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This comment was automatically generated based on the OWNER files in the repository.

[rootfs]

@cryo-zd nice catch. Would you like to take one step further to add go vet in the CI

[cryo-zd]

@cryo-zd nice catch. Would you like to take one step further to add go vet in the CI

My pleasure. I plan to introduce a separate vet target in the Makefile, and then make make test depend on it.

Does this approach sound reasonable to you?

[rootfs]

sounds good!