Add Grok 4.5 model support to xAI provider

[navedmerchant]

Name of model: Grok 4.5 (grok-4.5)
Model creator: xAI (SpaceXAI)
Link to model: https://docs.x.ai/developers/grok-4-5

Model Details

Grok 4.5 was publicly released on July 9, 2026. It is xAI's flagship model built for coding, agentic tasks, and knowledge work.

Property	Value
Model name	grok-4.5
Context window	500,000 tokens
Input price	$2.00 / 1M tokens
Cached input	$0.50 / 1M tokens
Output price	$6.00 / 1M tokens
Reasoning	Low, medium, or high (default high)
APIs	Responses API, Chat Completions
Modalities	Text, Image

Source: xAI Docs - grok-4.5, xAI Pricing

What needs to change

1. Add grok-4.5 model definition in packages/types/src/providers/xai.ts with the correct context window, pricing, and reasoning support.
2. Update xaiDefaultModelId from grok-4.20 to grok-4.5 (it's the new flagship).
3. Fix reasoning format bug in src/api/providers/xai.ts — the xAI Responses API expects reasoning: { effort: "high" } but the handler was passing reasoning: { reasoning_effort: "high" } (Chat Completions format). This latent bug affects grok-3-mini and would break grok-4.5's configurable reasoning.
4. Update tests to cover the reasoning format fix and grok-4.5 reasoning support.