Fix message memoization, stop button, anthropic

[bfischer1121]

Why

Fix Messages memoization and Stop button functionality after recent switch from isLoading to status.
Fix this error when using Claude 3.7 (and possibly other providers):

invalid request error: all messages must have non-empty content except for the optional final assistant message

How

• Revert React.memo condition from almost always true statement to prior logic.
• Show StopButton when status is streaming
• Copy text parts to content field before being sent to provider

[vercel]

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