feat(tokens): add in-process text token counting

[localai-org-maint-bot]

Summary

• add a public pkg/tokens helper for approximate in-process token counting
• resolve known tiktoken model encodings with a conservative cl100k_base fallback
• count plain and multimodal text content while rejecting unsupported shapes
• promote tiktoken-go to a direct dependency and add hermetic Ginkgo coverage

Testing

• go test -count=1 ./pkg/tokens
• go vet ./pkg/tokens
• gofmt -l pkg/tokens/*.go
• git diff --check origin/master...HEAD

Documentation

No user-facing runtime behavior or configuration surface changes; the new API is documented with Go comments.

Closes #9537

[mudler]

@localai-org-maint-bot code isn't used. I don't like to introduce code which is not used