Add Ollama Cloud provider with native Ollama API support

[gs-deliverists-io]

 - Implements Ollama Cloud provider using native Ollama API
 - API base: https://ollama.com (matching Python client)
 - Uses OLLAMA_API_KEY environment variable
 - Supports cloud models with -cloud suffix
 - Implements /api/tags, /api/chat, /api/embed endpoints
 - Adds assume_models_exist? to allow unregistered cloud models

 ## What this does

 This PR allows to use ollama-cloude provider for chatting.

 ## Type of change

 - [x] New feature
 - [ ] Documentation

 ## Scope check

 - [x] I read the [Contributing Guide](https://github.com/crmne/ruby_llm/blob/main/CONTRIBUTING.md)
 - [x] This aligns with RubyLLM's focus on **LLM communication**
 - [x] This isn't application-specific logic that belongs in user code
 - [x] This benefits most users, not just my specific use case

 ## Required for new features

 - [ ] I opened an issue **before** writing code and received maintainer approval
 - [ ] Linked issue: #___

 **PRs for new features or enhancements without a prior approved issue will be closed.**

 ## Quality check

 - [ ] I ran `overcommit --install` and all hooks pass
 - [ ] I tested my changes thoroughly
   - [ ] For provider changes: Re-recorded VCR cassettes with `bundle exec rake 'vcr:record[provider_name]'`
   - [ ] All tests pass: `bundle exec rspec`
 - [ ] I updated documentation if needed
 - [ ] I didn't modify auto-generated files manually (`models.json`, `aliases.json`)

 ## AI-generated code

 - [ ] I used AI tools to help write this code
 - [ ] I have reviewed and understand all generated code (required if above is checked)

 ## API changes

 - [ ] Breaking change
 - [ ] New public methods/classes
 - [ ] Changed method signatures
 - [ ] No API changes

Example of usage

require "ruby_llm"

RubyLLM.configure do |config|
  config.ollama_api_key = "your-api-key"
end

#Use with cloud models (note the -cloud suffix)
chat = RubyLLM.chat model: "gpt-oss:120b-cloud", provider: :ollama_cloud
response = chat.ask "What is Ruby?"
puts response.conten

[crmne]

We have already support for different API bases and #612 will add support for API keys.

Also this is violating the guidelines:

Required for new features

• I opened an issue before writing code and received maintainer approval
• Linked issue: #___

PRs for new features or enhancements without a prior approved issue will be closed.

Closing.