feat: implement batch classification API

[OneZero-Y]

What type of PR is this?

Feature - implement batch classification API

What this PR does / why we need it:

This PR implements batch classification API functionality, resolving the API completeness issue where POST /api/v1/classify/batch returned HTTP 501 "Not Implemented" error.

Changes:

1. Add batch classification API: Support processing multiple texts in a single request (configurable batch size)
2. Implement configurable processing: Sequential for small batches, concurrent for large batches
3. Add resource control: Configurable concurrency limits to prevent system overload

Configuration Examples:

Small batch request (sequential processing):

{
  "texts": ["solve math equation", "write business plan", "chemistry experiment"]
}

Large batch request (concurrent processing):

{
  "texts": [
    "solve differential equation", "business strategy analysis", 
    "chemistry reaction", "physics calculation", "market research",
    "mathematical modeling", "financial planning", "scientific experiment"
  ],
  "options": {"return_probabilities": true}
}

Configuration Examples:

# API Configuration
api:
  batch_classification:
    max_batch_size: 100          # Maximum number of texts in a single batch
    concurrency_threshold: 5     # Switch to concurrent processing when batch size > this value
    max_concurrency: 8           # Maximum number of concurrent goroutines

Which issue(s) this PR fixes:

Fixes: implement batch classification API

[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/api/server_test.go
• src/semantic-router/pkg/api/server.go
• src/semantic-router/pkg/config/config.go

📁 config

Owners: @rootfs
Files changed:

• config/config.yaml

📁 website

Owners: @Xunzhuo
Files changed:

• website/docs/api/classification.md

---

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

[rootfs]

@OneZero-Y thanks for contributing again! Would you please run pre-commit run --all-files? Maybe you can open another PR to add this to CONTRIBUTING.md

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[rootfs]

@OneZero-Y can you share your local test with batch classify and potentially add this to the doc too?

[Xunzhuo]

Thanks for working on it, please update the classify API docs.