fix: nil-safe content.strip in chat_completion final-response paths

[obie]

Symptom

NoMethodError: undefined method 'strip' for nil
  from lib/raix/chat_completion.rb:226

A long-running chat completion crashed with the above when the LLM returned a final assistant message containing "content": null. The same input on retry succeeded (the model returned non-nil content the second time), confirming the bug is non-deterministic and depends on what the provider emits.

When it happens

Some providers — notably Gemini under certain stop conditions (length-limit hit, tool-exhaustion fallback, etc.) — return:

{ "choices": [ { "message": { "content": null, "tool_calls": [] } } ] }

Raix::ChatCompletion calls content.strip on the final response in three places without guarding against nil:

• lib/raix/chat_completion.rb:175 (max_tool_calls exceeded path)
• lib/raix/chat_completion.rb:218 (stop_tool_calls_and_respond! path)
• lib/raix/chat_completion.rb:226 (no tool_calls fall-through path)

When content is nil, nil.strip raises NoMethodError and the whole completion blows up.

Fix

Change content.strip → content.to_s.strip at each of the three call sites. nil.to_s is "", so a nil response now coerces to an empty string instead of raising. No other behavior changes — non-nil content still strips identically.

Tests

Added spec/raix/nil_content_spec.rb covering each of the three branches by stubbing ruby_llm_request to return {"choices" => [{"message" => {"content" => nil, ...}}]}:

• Plain final response with nil content (line 226 path)
• Forced final response after max_tool_calls exceeded with nil content (line 175 path)
• Forced final response after stop_tool_calls_and_respond! with nil content (line 218 path)

Each spec verifies chat_completion returns "" instead of raising NoMethodError. Confirmed the specs fail on main and pass with the fix applied.

bundle exec rake runs clean: 96 examples, 0 failures, 37 files rubocop-clean.