Python: fix(core): align in-memory checkpoint isolation

[Patel Namraa (Namraa310806)]

Motivation & Context

This PR fixes checkpoint state isolation issues tracked in #7683.

Checkpoint state could previously share mutable references with the live workflow
state or with the internal InMemoryCheckpointStorage representation. As a
result, mutating a restored or loaded checkpoint could silently modify the
original checkpoint snapshot.

This is especially problematic for nested mutable values such as lists and
dictionaries because the mutation happens without an exception and without
another save() operation.

Description & Review Guide

• What are the major changes?

  • Isolate nested values when exporting and importing State using the existing
    checkpoint-value serialization mechanism.
  • Make InMemoryCheckpointStorage return caller-owned checkpoint copies using
    copy.deepcopy() for load(), list_checkpoints(), and get_latest().
  • Keep save() isolated so changes to the caller's checkpoint after saving
    cannot modify stored state.
  • Document the ownership contract on the CheckpointStorage protocol.
  • Add checkpoint-storage conformance tests covering both in-memory and file
    storage backends.
  • Add regression tests covering nested state and checkpoint data isolation.

• What is the impact of these changes?

  • Checkpoints remain stable snapshots after they are exported, restored, loaded,
    or listed.
  • Mutating a returned in-memory checkpoint no longer mutates the stored
    checkpoint.
  • The direct State export/import path is also isolated, including nested
    mutable values.
  • This does not change the public checkpoint value contract or workflow
    behavior; it only prevents unintended reference sharing.

• What do you want reviewers to focus on?

  • The ownership/isolation boundaries between State, WorkflowCheckpoint,
    and CheckpointStorage.
  • The use of the checkpoint-value isolation mechanism for State, while
    InMemoryCheckpointStorage uses copy.deepcopy() as requested for the
    in-memory backend.
  • The conformance tests verifying that both in-tree storage backends satisfy
    the checkpoint ownership contract.

Related Issue

Fixes #7683

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

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[Tao Chen (TaoChenOSU)]

Superseded by #7847

[Patel Namraa (Namraa310806)]

Tao Chen (@TaoChenOSU)
Before the PR was superseded, I updated my implementation to incorporate the
feedback and address both related isolation issues:

• State.export_state() / import_state() retain checkpoint-value isolation
  so the original Python: [Bug]: Checkpoint state is not isolated from live workflow state across restoration and storage boundaries #7683 aliasing issue is fixed without unnecessarily changing
  the checkpoint value contract.
• InMemoryCheckpointStorage uses copy.deepcopy() for caller/storage
  isolation, rather than pickle, following the feedback that pickle is not
  needed for the in-memory backend.
• The CheckpointStorage ownership contract is documented explicitly.
• The conformance tests cover both InMemoryCheckpointStorage and
  FileCheckpointStorage, including nested mutable checkpoint data.
• Regression coverage was added for the State export/import isolation boundary.

Since this PR is now superseded by #7847, I won't duplicate the work. If any of
these implementation details or tests are useful for the consolidated PR,
please feel free to incorporate them.