Cosmos: Use Driver to implement Read Item

[analogrelay]

In preparation for #3906 , let's migrate the ContainerClient::read_item call to use the driver. It's a fairly simple, but highly-used API that will allow us to get initial feedback on issues with the driver model.

[github-actions]

🎯 Agentic Issue Triage

This is an internal task to migrate ContainerClient::read_item to use the new Cosmos driver, as part of the larger effort tracked in #3906.

Analysis

Type: Feature / Internal task
Crate: azure_data_cosmos
Area: Cosmos SDK driver migration

ContainerClient::read_item is a heavily-used, relatively simple API, making it a good first candidate for driver integration. Migrating it will provide early feedback on the driver model before tackling more complex operations.

Relevant context

• Parent tracking issue: Cosmos: Switch SDK over to using the driver #3906 "Cosmos: Switch SDK over to using the driver" — scope includes constructing CosmosOperations, executing them via the driver, and wrapping CosmosResponse with deserialization + diagnostics/header access.
• The driver skeleton is already in place (per Cosmos: Switch SDK over to using the driver #3906), so this work can begin.
• No linked PR yet; no duplicate open issues found.

Implementation hints

Key steps likely involved:

• Construct a CosmosOperation representing a ReadItem request in the driver layer.
• Execute it via the driver's transport interface.
• Wrap the returned CosmosResponse to expose body deserialization (Response<T>), diagnostics, and response headers — consistent with the existing read_item return type.
• Remove or bypass the legacy transport path for this operation.
• Run existing read_item integration tests (with recordings) to confirm correctness: cargo test -p azure_data_cosmos --test <read-item-test> --all-features.

Relevant files to look at:

• sdk/cosmos/azure_data_cosmos/src/clients/container_client.rs — current read_item implementation
• sdk/cosmos/azure_data_cosmos_driver/ — driver skeleton
• sdk/cosmos/azure_data_cosmos/src/ — response wrappers and options

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