fix: expose sandbox error retryability

[qiyaoq-oai]

Context: temporalio/sdk-python#1548

This pull request adds provider-neutral retryability metadata for sandbox failures.

• Adds SandboxError.retryable with True, False, and None for unknown classification.
• Propagates retryability through wrapped sandbox errors.
• Emits retryability in sandbox finish events and trace error payloads.
• Classifies clear terminal/transient cases across sandbox providers without exposing provider internals to integrations.
• Adds focused tests for core error behavior, provider classifiers, and observability.

[qiyaoq-oai]

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[chatgpt-codex-connector]

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[seratch]

One non-blocking integration note for the Temporal follow-up: it would be safer to treat only retryable is False as an explicit non-retryable signal, while preserving the orchestrator's existing policy for True and None. In particular, True should not be interpreted as a blanket guarantee that replaying an arbitrary exec is side-effect safe.