fix: fix processing null field

[ninan-nn]

Summary

• restore lifecycle route serialization to omit None fields in JSON responses instead of emitting explicit null
• align the lifecycle OpenAPI contract so response-side optional fields are omitted when unset, while request-side nullable inputs that are still intentionally supported remain nullable
• update route tests and lifecycle docs to reflect the no-null response contract used for backward compatibility with released SDKs

What This Fixes

This PR addresses the lifecycle compatibility issue behind issue #550.

Before this change, POST /sandboxes, GET /sandboxes/{id}, and GET /sandboxes could serialize optional response fields such as metadata, expiresAt, status.reason, status.message, and status.lastTransitionAt as explicit JSON null values.

That behavior broke older released Python SDK versions which distinguished between:

• field omitted
• field explicitly present with null

In particular, metadata: null could trigger client-side parsing failures during sandbox creation.

After this change:

• lifecycle responses omit unset optional fields instead of returning null
• manual-cleanup sandboxes are represented by an omitted expiresAt field in responses
• request-side nullable inputs that are still part of the API surface, such as timeout and image.auth, remain documented as nullable in the spec

Testing

• cd server && uv run pytest tests/test_routes_create_delete.py tests/test_routes_get_sandbox.py tests/test_routes_list_sandboxes.py
• passed: 16 passed

[chatgpt-codex-connector]

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[Pangjiping]

LGTM

[hittyt]

LGTM