Fix NoMethodError when Error is raised with a string argument

[cgmoore120]

Summary

RubyLLM::Error.new("message") puts the string in the response parameter, then super(message || response&.body) calls .body on a String, raising NoMethodError. This fix detects a String in the first position and treats it as the message instead, keeping full backward compatibility with the two-arg ErrorMiddleware path.

Reproduction

1. Install ruby_llm and open a console:

require 'ruby_llm'

2. Raise an error with just a string message (as several providers already do internally):

RubyLLM::Error.new("something went wrong")

3. Expected: An Error instance with message == "something went wrong" and response == nil

4. Actual:

NoMethodError: undefined method `body' for an instance of String
  from lib/ruby_llm/error.rb:11:in `initialize'

This happens because the constructor signature is initialize(response = nil, message = nil) — the string lands in response, and super(message || response&.body) calls .body on it.

Existing call sites that trigger this

lib/ruby_llm/providers/bedrock/streaming.rb:49      → raise Error, "..."
lib/ruby_llm/providers/gemini/images.rb:32           → raise Error, "..."
lib/ruby_llm/providers/vertexai.rb:62                → raise Error, "..."
lib/ruby_llm/providers/anthropic/embeddings.rb:11    → raise Error "..."

The normal ErrorMiddleware path always passes both args (response, message) and is unaffected.

Test plan

• Error.new("msg") no longer raises NoMethodError
• Error.new("msg").message returns the string
• Error.new("msg").response is nil
• Two-arg path Error.new(response, "msg") still works
• No-arg path Error.new still works
• Subclasses (BadRequestError, etc.) inherit the fix
• Existing ErrorMiddleware specs pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.51%. Comparing base (bac3f3f) to head (eb3345d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #653      +/-   ##
==========================================
+ Coverage   81.50%   81.51%   +0.01%     
==========================================
  Files         116      116              
  Lines        5476     5479       +3     
  Branches     1430     1432       +2     
==========================================
+ Hits         4463     4466       +3     
  Misses       1013     1013              

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