Move GenericStringModel to warp_server_client.

[vorporeal]

Description

Moves the portable generic-string scaffolding into warp_server_client as the next conservative step in the cloud-object extraction stack. Now that GenericCloudObject lives upstream, GenericStringModel, Serializer, and their portable helpers can move with it without pulling concrete model behavior out of the app crate.

This intentionally keeps the app-local pieces in warp: StringModel, CloudStringObject, CloudModelType, ModelEvent, QueueItem, ObjectClient dispatch, Drive rendering, and concrete model types. The only additional upstream hook is serializer-provided object-format metadata, which lets warp_server_client own the generic ServerObjectModel implementation without depending on app traits.

Testing

• cargo fmt --manifest-path /Users/david/src/warp/Cargo.toml --all
• cargo check --manifest-path /Users/david/src/warp/Cargo.toml -p warp_server_client
• cargo check --manifest-path /Users/david/src/warp/Cargo.toml -p warp
• git -C /Users/david/src/warp --no-pager diff --check

Co-Authored-By: Oz oz-agent@warp.dev

[vorporeal]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Extract cloud object persistence logic to other crates. #11179
• Extract CloudObject client and models to separate crates. #11166
• Create cloud_objects crate. #11165
• Move cloud object models to separate crate. #11164
• Move GenericStringModel to warp_server_client. #11117 👈 (View in Graphite)
• Move GenericCloudObject to warp_server_client. #11115
• Move GenericServerObject to warp_server_client. #11114
• Move some types to the warp_server_client crate. #11113
• Delete unused new_from_server_update function. #11112
• master

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