Pausing Phase for Sandbox

[rishisulakhe]

Ⅰ. Describe what this PR does

Added a Pausing intermediate phase to the Sandbox lifecycle so users can distinguish between "hibernation in progress" and "hibernation complete":

Before: Running -> Paused (pod may still be deleting)
After: Running -> Pausing (pod being deleted) -> Paused (hibernation complete)

Ⅱ. Does this pull request fix one issue?

Fixees #250

Ⅲ. Describe how to verify it

Unit tests(passing)

go test ./pkg/controller/sandbox/... -v -run "TestCalculateStatus"
go test ./pkg/utils/sandboxutils/... -v
go test ./pkg/servers/e2b -v -run "TestPauseSandbox|TestConnectSandbox|TestResumeSandbox"

Ⅳ. Special notes for reviews

[kruise-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[kruise-bot]

@rishisulakhe: PR needs rebase.

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