fix(studio): upgrade managed sidecar CLI to 0.52.18

[zyn080302]

Summary

• Pin the VeADK-managed AgentKit CLI to v0.52.18 with verified SHA256 values for all five supported platforms.
• Keep the Studio Release Server manifest allowlist and embedded linux-x64 archive checksum aligned with the VeADK pin.
• Add a contract regression test so future Release Server and VeADK CLI pins cannot drift silently.

Why

AgentKit CLI v0.52.18 contains managed Sidecar support for Volcengine cn-beijing. Without synchronizing the Release Server constants, Studio Bundle publication/consumption still rejects or embeds the previous v0.52.14 artifact.

Compatibility

• Does not automatically update, restart, or republish existing Studios or Agents.
• The new CLI is used only when an explicit managed Sidecar create/update/publish flow is executed.
• Ordinary non-Sidecar deployment remains on the AgentKit Python SDK path.
• This PR does not deploy Studio or Release Server resources.

Validation

• Full Python suite: 3811 passed, 6 skipped, 2 xfailed.
• Focused managed Sidecar/Studio/Release Server regression: 410 passed.
• pre-commit: Ruff check, Ruff format, hardcoded-secret scan passed.
• git diff --check passed.