Preserve sandbox storageMb during apply

[mstolarzblaxelai]

• Preserves storageMb when applying sandbox runtime config before typed SDK support lands.
• Adds storageMb to generated sandbox config samples.
• Covers runtime field preservation and sample rendering.

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Note

Adds storageMb to the list of extra runtime fields preserved during sandbox apply, adds a commented-out example in the TOML sample config, a new YAML sample file, and test coverage for both.

^{Written by Mendral for commit e276c9d.}

[mendral-app]

LGTM

The change is minimal and correct: it appends "storageMb" to the allowlist in preserveExtraRuntimeFields, which uses a safe map-lookup pattern. The test confirms the field round-trips through the preservation logic. No security, correctness, or data-loss concerns in this diff.

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