fix(anthropic): instrument anthropics system message as gen_ai.prompt.0

[dinmukhamedm]

Address #2187 by dumping system message as gen_ai.prompt.0.content and gen_ai.prompt.0.role = "system". This will also increment all subsequent message indices by 1, e.g. first user message will become gen_ai.prompt.1.

• I have added tests that cover my changes. <-- modified the existing ones with system message to check for it
• If adding a new instrumentation or changing an existing one, I've added screenshots from some observability platform showing the change.
• PR name follows conventional commits format: feat(instrumentation): ... or fix(instrumentation): ....
• (If applicable) I have updated the documentation accordingly.

[nirga]

Nice, thanks @dinmukhamedm!