[simonw/llm] Update retired GPT-5 snapshot docs (issue #1617)

Job description

Fix LLM issue 1617: https://github.com/simonw/llm/issues/1617 Replace the retiring gpt 5 2025 08 07 snapshot in the identified documentation examples with the current supported model identifier, keeping the examples internally consistent. Run the focused documentation checks used by the repository. Deliverables: upstream PR linked to the issue, changed files summary, and validation notes. The PR body or a linked issue comment must mention this OpenJobs listing URL.