fix(Gemini): Add Usage to StepFinishEvent

[Plytas]

Description

When using ->withTools()->asStream() in Gemini, we're missing token usage on the intermediate LLM call:

  User Message
       │
       ▼
  ┌─────────┐    usage: prompt=100, comp=200, thought=50
  │ LLM #1  │──→ StepFinishEvent  ❌ no usage field
  └────┬────┘
       │ tool calls
       ▼
  ┌──────────┐
  │ Tool Exe │
  └────┬─────┘
       │ tool results
       ▼
  ┌─────────┐    usage: prompt=200, comp=400, thought=0
  │ LLM #2  │──→ StepFinishEvent  ❌ no usage field
  └────┬────┘
       │ final text
       ▼
  StreamEndEvent
    usage: prompt=200, comp=400  ← ❌ only LLM #2

There is a code block in Gemini Stream handler that supposedly updates usage to include previous usage:

prism/src/Providers/Gemini/Handlers/Stream.php

Lines 359 to 367 in 5d6cc65

	if ($previousUsage instanceof Usage && $this->state->usage() instanceof Usage) {
	$this->state->withUsage(new Usage(
	promptTokens: $previousUsage->promptTokens + $this->state->usage()->promptTokens,
	completionTokens: $previousUsage->completionTokens + $this->state->usage()->completionTokens,
	cacheWriteInputTokens: ($previousUsage->cacheWriteInputTokens ?? 0) + ($this->state->usage()->cacheWriteInputTokens ?? 0),
	cacheReadInputTokens: ($previousUsage->cacheReadInputTokens ?? 0) + ($this->state->usage()->cacheReadInputTokens ?? 0),
	thoughtTokens: ($previousUsage->thoughtTokens ?? 0) + ($this->state->usage()->thoughtTokens ?? 0)
	));
	}

However, this is effectively dead code as StreamEndEvent is fired before this and there's no way to retrieve this updated usage. There are no tests that hit this block.

Proposed solution

I propose to expose Usage via StepFinishEvent. This PR only adds it to Gemini provider (as that's what I have access to), but I'd be able to add it to all instances of StepFinishEvent if needed.