MEAI: Update Experimental / Preview Features

[jeffhandley]

1. Remove [Experimental] from IChatReducer interface, leaving the attribute on the concrete chat reducer implementations
2. Remove the prerelease label from the Microsoft.Extensions.AI.OpenAI package
3. Apply [Experimental] to the MEAI.OpenAI APIs that utilize experimental OpenAI APIs (Responses, Assistants, and Real-Time). Those attribute propagate the OPENAI001 and OPENAI002 diagnostic Ids rather than requiring consumers to suppress both and OPENAI diagnostic and an MEAI diagnostic.

As part of changing API experimental annotations, API Chief was run to update the baselines for the MEAI.Abstractions and MEAI.OpenAI libraries. A lot of the differences in the MEAI.Abstractions baseline difference are ordering of the APIs or adding in APIs that were introduced as experimental earlier, getting the baseline up to date with what the tool generates.

Marking this PR as draft for a couple reasons:

1. We still want to take another pass over the MEAI.OpenAI API surface to ensure we're ready to remove the prerelease label and stabilize the APIs not annotated as [Experimental]
2. It's worth discussing whether the OpenAI experimentals should reuse OPENAI001 and OPENAI002 or use MEAI001.

Microsoft Reviewers: Open in CodeFlow