Add volume mounts

[Dingway98]

Note

Medium Risk
Adds new client surface area and wire serialization for volume mounts plus new /volume API calls; risk is mainly around API contract/serialization mismatches rather than core runtime behavior.

Overview
Adds first-class volume support to the SDK: new client.volumes managers (sync/async) with create, list, and get calling the /volume endpoints, plus new Volume/CreateVolumeParams models.

Extends sandbox creation to support mounting volumes via CreateSandboxParams.mounts and SandboxVolumeMount (with rw/ro), updates exports/docs, and adds wire-contract tests for both mounts and volumes. Also includes minor formatting-only test tweaks and bumps version to 0.90.2.

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