Added proper handling of streaming error responses across both Faraday V1 and V2

[dansingerman]

What this does

When used within our app, streaming error responses were throwing an error and not being properly handled

worker      | D, [2025-07-03T18:49:52.221013 #81269] DEBUG -- RubyLLM: Received chunk: event: error
worker      | data: {"type":"error","error":{"details":null,"type":"overloaded_error","message":"Overloaded"}               }
worker      | 
worker      | 
worker      | 2025-07-03 18:49:52.233610 E [81269:sidekiq.default/processor chat_agent.rb:42] {jid: 7382519287f08cfa7cd1e4e4, queue: default} Rails -- Error in ChatAgent#send_with_streaming: NoMethodError - undefined method `merge' for nil:NilClass
worker      | 
worker      |       error_response = env.merge(body: JSON.parse(error_data), status: status)
worker      |                           ^^^^^^
worker      | 2025-07-03 18:49:52.233852 E [81269:sidekiq.default/processor chat_agent.rb:43] {jid: 7382519287f08cfa7cd1e4e4, queue: default} Rails -- Backtrace: /Users/dansingerman/.rbenv/versions/3.1.6/lib/ruby/gems/3.1.0/gems/ruby_llm-1.3.1/lib/ruby_llm/streaming.rb:91:in `handle_error_chunk'
worker      | /Users/dansingerman/.rbenv/versions/3.1.6/lib/ruby/gems/3.1.0/gems/ruby_llm-1.3.1/lib/ruby_llm/streaming.rb:62:in `process_stream_chunk'
worker      | /Users/dansingerman/.rbenv/versions/3.1.6/lib/ruby/gems/3.1.0/gems/ruby_llm-1.3.1/lib/ruby_llm/streaming.rb:70:in `block in legacy_stream_processor'
worker      | /Users/dansingerman/.rbenv/versions/3.1.6/lib/ruby/gems/3.1.0/gems/faraday-net_http-1.0.1/lib/faraday/adapter/net_http.rb:113:in `block in perform_request'
worker      | /Users/dansingerman/.rbenv/versions/3.1.6/lib/ruby/gems/3.1.0/gems/net-protocol-0.2.2/lib/net/protocol.rb:535:in `call_block'
worker      | /Users/dansingerman/.rbenv/versions/3.1.6/lib/ruby/gems/3.1.0/gems/net-protocol-0.2.2/lib/net/protocol.rb:526:in `<<'
worker      | /Users/dansingerman/.rbenv/versions/3.1.6/lib/ruby/gems/3.1.0/gems/net-protocol-0.2.2/lib/net/protocol.rb

It looks like the introduction of support for Faraday V1 introduced this error, as the error handling relies on an env that is no longer passed. This should provide a fix for both V1 and V2.

One thing to note, I had to manually construct the VCR cassettes, I'm not sure of a better way to test an intermittent error response.

I have also only written the tests against anthropic/claude-3-5-haiku-20241022 - it's possible other models with a different error format may still not be properly handled, but even in that case it won't error for the reasons fixed here.

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation
• Performance improvement

Scope check

• I read the Contributing Guide
• This aligns with RubyLLM's focus on LLM communication
• This isn't application-specific logic that belongs in user code
• This benefits most users, not just my specific use case

Quality check

• I ran overcommit --install and all hooks pass
• I tested my changes thoroughly
• I updated documentation if needed
• I didn't modify auto-generated files manually (models.json, aliases.json)

API changes

• Breaking change
• New public methods/classes
• Changed method signatures
• No API changes

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