feat(event-schema): Updating several gen_ai attributes to their latest representation

[constantinius]

Follow up for #5797

Updates the following fields to their newest representation:

• gen_ai.system -> gen_ai.provider.name
• gen_ai.prompt -> gen_ai.input.messages
• gen_ai.request.messages -> gen_ai.input.messages
• gen_ai.response.text -> gen_ai.output.messages
• gen_ai.response.tool_calls -> gen_ai.output.messages
• gen_ai.request.available_tools -> gen_ai.tool.definitions
• gen_ai.tool.input -> gen_ai.tool.call.arguments
• gen_ai.tool.output -> gen_ai.tool.call.result
• gen_ai.response.finish_reason -> gen_ai.response.finish_reasons

Now properly aligns gen_ai.request.messages/gen_ai.input.messages and gen_ai.response.text/gen_ai.output.messages.

Closes https://linear.app/getsentry/issue/TET-2188/mark-gen-aiinputmessages-as-pii-maybe

[linear-code]

TET-2188 Mark `gen_ai.input.messages` as pii maybe

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 10f4b72. Configure here.}