feat(routing): Implement in-tree keyword-based routing

[srini-abhiram]

What this PR does / why we need it:

• Provides a fast-path classification mechanism using keyword matching. This allows the router to bypass expensive model inference for simple, predefined routes.
• The new KeywordClassifier supports AND, OR, and NOR operators with configurable case sensitivity.
• It is executed before model-based classifiers to ensure an early exit on a match.
• Rules are defined in the main configuration file under a new keyword_rules section.
• Add Unit tests for the keyword classifier.

Which issue(s) this PR fixes:
Fixes #364

Unit tests results: Passed

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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/utils/classification/benchmark_test.go
• src/semantic-router/pkg/utils/classification/keyword_classifier.go
• src/semantic-router/pkg/utils/classification/keyword_classifier_test.go
• src/semantic-router/pkg/config/config.go
• src/semantic-router/pkg/utils/classification/classifier.go

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🎉 Thanks for your contributions!

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

[srini-abhiram]

I hope the implementation is as per issue. I have assumed most of it, please correct me if Im wrong.

[rootfs]

@srini-abhiram thanks! can you sign DCO and fix pre-commit?

Ensure you have a local copy of your branch by [checking out the pull request locally via command line](https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/checking-out-pull-requests-locally).
In your local branch, run: git rebase HEAD~2 --signoff
Force push your changes to overwrite the branch: git push --force-with-lease origin issue-364

[Copilot]

Pull Request Overview

This PR implements a keyword-based routing mechanism that provides fast-path classification before falling back to model-based inference. The keyword classifier supports AND, OR, and NOR logical operators with configurable case sensitivity, allowing the router to quickly match simple, predefined routes without expensive model inference.

Key Changes:

• Introduced KeywordClassifier with support for AND, OR, and NOR operators
• Modified the classification pipeline to check keyword rules before model-based classification
• Added configuration support for keyword rules in the router config

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
keyword_classifier.go	Implements the core keyword matching logic with operator support
keyword_classifier_test.go	Unit tests covering all operator types and case sensitivity
benchmark_test.go	Performance benchmarks for keyword classification
classifier.go	Integrates keyword classifier into the classification pipeline
config.go	Adds KeywordRule struct and configuration field

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[srini-abhiram]

I have signed off the wrong commits, let me put the changes in a fresh branch and create a new PR