Expose generation index to tool callables in GRPOTrainer

[lukehinds]

What does this PR do?

When using GRPOTrainerwith tools, the trainer generates multiple completions per prompt (num_generations), then executes tool calls for each completion via _tool_call_loop. However, tool callables have no way to know which completion triggered them.

The fix exposes the completion index to tool callables by inspecting each tool's signature at initialization and caching whether it accepts a _completion_idx parameter. During tool execution in _tool_call_loop, if a tool declares this parameter, the trainer passes the current completion index; otherwise it calls the tool normally. This allows tools to know which of the multiple generated completions triggered them, enabling per-completion attribution for reward functions without breaking existing tools that don't need this information.

Fixes #4866

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