feat(sql_connect): merge "X-Client-Platform" header into "X-Client-Version"

[dconeybe]

This PR updates the gRPC metadata in firebase_data_connect by merging the X-Client-Platform header into the X-Client-Version header (formatting as flutter/<version>) and removing the standalone X-Client-Platform header.

This is an update to PR #18484 which was merged yesterday and added both of these headers. This update was made to accommodate the firebase-js-sdk because the "X-Client-Platform" header causes the CORS OPTIONS preflight request to fail with a 403 "Permission Denied" error due to that header being absent from Google's list of whitelisted CORS headers. See firebase/firebase-js-sdk#10217 for a few more details, if interested. Note that this exact problem affects this flutter sdk when running in a web browser, so the fix is critical here in this repository too.

Highlights

• Header Consolidation: Merged the platform identifier into the X-Client-Version header (e.g. flutter/<version>) and removed the separate X-Client-Platform (x-client-platform) header.
• Test Updates: Updated test suites to verify the new X-Client-Version header value format.

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[stephenarosaj]

LGTM