Python: Preserve checkpoint ancestry when workflows resume

[davidahmann]

Problem
Resuming a workflow from checkpoint dropped the restored checkpoint from the ancestry chain of newly-created checkpoints, which breaks deterministic checkpoint lineage after restart.

Why now
Issue #4588 tracks resume correctness. Without this, resumed runs lose the audit trail needed to explain which checkpoint produced the next persisted state.

What changed

• remember the restored checkpoint id on resume and seed the next checkpoint's previous_checkpoint_id with it
• clear the resume parent after successful completion
• add a regression test covering resumed checkpoint chaining

Validation

• uv run pytest packages/core/tests/workflow/test_checkpoint.py -k 'test_workflow_checkpoint_chaining_via_previous_checkpoint_id or test_resumed_workflow_keeps_previous_checkpoint_id_chain'

Refs #4588

[davidahmann]

User impact: Resumed workflows now preserve checkpoint ancestry so the next persisted checkpoint points back to the restored checkpoint instead of dropping the chain.
Change: The runner remembers the restored checkpoint ID until the first resumed checkpoint is written, and the new regression test covers the resumed chain explicitly.
Validation:

• uv run pytest packages/core/tests/workflow/test_checkpoint.py -k 'test_workflow_checkpoint_chaining_via_previous_checkpoint_id or test_resumed_workflow_keeps_previous_checkpoint_id_chain' passed

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