Stop defaulting temperature in Raix::Configuration

[obie]

Summary

• Raix::Configuration#initialize no longer sets temperature = 0.0. Callers who want a specific temperature now have to set one explicitly (or rely on the provider's server-side default).
• Adds spec/raix/default_temperature_spec.rb covering both directions: the parameter is now omitted when unset, and an explicit value (including class-level configure) still flows through to RubyLLM::Chat#with_temperature.
• CHANGELOG entry under `[Unreleased]`.

Why

Production hit RubyLLM::Error: No endpoints found that can handle the requested parameters from a routine chat_completion(json: true) call against `anthropic/claude-opus-4-7`. Sentry: INGENIUM-6.

Tracing through Raix + RubyLLM + OpenRouter:

1. `Configuration#initialize` ran `self.temperature = DEFAULT_TEMPERATURE` (`0.0`).
2. `chat_completion` line: `params[:temperature] ||= temperature.presence || configuration.temperature` → `params[:temperature] = 0.0`.
3. `ruby_llm_request` line: `chat.with_temperature(params[:temperature]) if params[:temperature]` — `0.0` is truthy in Ruby, so it fires.
4. RubyLLM's OpenRouter provider serializes `temperature: 0.0` into the payload.
5. `json: true` also sets `provider.require_parameters: true` (only-route-to-providers-that-natively-support-every-param).
6. OpenRouter's catalog for `anthropic/claude-opus-4.7` lists `supported_parameters` as: `include_reasoning, max_tokens, reasoning, response_format, stop, structured_outputs, tool_choice, tools, verbosity` — no `temperature`.
7. With `require_parameters: true` and no eligible providers → 404 "No endpoints found...".

Verified locally with the exact production payload via `curl`: removing the `temperature: 0.0` field is the difference between success and the 404. Repro'd in Raix and confirmed the fix unblocks it.

Temperature is the only knob the Configuration was forcing on. Every other parameter (`top_p`, `frequency_penalty`, `presence_penalty`, etc.) is nil by default and is only serialized when the caller sets it. This change makes temperature consistent with the rest.

Breaking change note

Requests that previously ran at temperature 0.0 will now run at the provider's default (typically ~1.0). Pin explicitly if you relied on the old behavior:

```ruby
Raix.configure { |c| c.temperature = 0.0 } # global

or

class MyAgent
include Raix::ChatCompletion
self.temperature = 0.0 # per-class
end
```

Test plan

• `bundle exec rspec spec/raix/default_temperature_spec.rb` — 6 examples, 0 failures
• `bundle exec rspec` (full suite) — 104 examples, 0 failures, 1 pending (pre-existing)
• End-to-end repro via `bin/rails runner` against `anthropic/claude-opus-4-7` with stock Raix → 404; with this patch → returns the JSON object cleanly