fix: isolate terminal plugin error snapshots - #866

Issue #, if available

Fixes #852.

Description of changes

Terminal operation hooks now copy the error's mutable stack-trace list at the two
existing assignments: _end_info for checkpointed operations and
on_child_context_end for child contexts that finish without a checkpoint.
A plugin clearing its snapshot cannot change stored error diagnostics, a later
snapshot, or the ChildContextError subsequently raised to the caller.

The runtime diff is exactly two error= lines. OperationInfo.from_operation
remains a staticmethod, and both terminal-info constructors keep their existing
field conversion and delivery behavior.

Design

• Design note (rule 3): not needed; this corrects snapshot ownership without
  changing the public API. The requested review revision narrows the change to
  the two existing error assignments.
• Simpler options considered (rule 2): wrap each existing error expression in
  _copy_error. This reuses the narrow list copy without introducing a factory
  refactor, subclass behavior change, deep copy, or scheduling changes.
• Root cause (rule 5): terminal snapshots shared the input ErrorObject's mutable
  stack-trace list. The virtual-child path also reuses the ErrorObject to raise
  ChildContextError after the terminal hook returns, so a plugin could change
  the user's exception even without a child checkpoint.
• Net runtime source change: two lines added / two removed; no API additions or
  changes.

Validation

• Existing regressions reproduce checkpointed terminal-error aliasing across
  STEP, CALLBACK, CHAINED_INVOKE, and CONTEXT, through natural and queued delivery.
• The added virtual-child unit/public-handler regressions failed on the previous
  PR head (235d8de): both observed a cleared original/raised stack trace. They
  pass with the second error-copy assignment.
• Focused regressions: 6 passed and 11 subtests passed.
• hatch run dev-core:test --tb=short: 2,121 passed and 24 subtests passed.
• hatch run dev-core:typecheck: no issues in 95 source files.
• Package hatch fmt --check, commit-message validation, and git diff --check:
  passed.
• Python 3.13.17 and CI-pinned Hatch 1.16.5.

Public-handler coverage verifies stored step failures replay without executing
step bodies and virtual-child failures reach the caller with their original
message/type/data/stack despite a mutating terminal plugin. Metadata, hook counts,
replay flags, and absent/empty error semantics remain covered.

Existing test update: test_builds_terminal_operation_info now supplies a mutable
stack and verifies snapshot mutation leaves the input error intact; its previous
field/delivery expectations remain. The earlier PR-added subclass-factory test
is replaced by terminal-info field preservation, matching the unchanged static
factory contract.

Assumptions

• Aliasing and error propagation are verified with real SDK error/plugin objects
  and the public durable handler. No new deployed-service assumption is needed
  for these ownership changes.
• Deployed Lambda validation of this revision is unverified. This PR does not
  change OTel rendering or testing-service behavior.

Decisions not covered by the issue or the design note

• Preserve the existing static converter as requested in review; subclass factory
  behavior is not changed by this fix.
• Keep isolation scoped to terminal hooks and invocation operation-map snapshots.
  Plugins receiving the same hook object still share that snapshot.

Checklist

• Both error-copy sites have regressions that fail without the copy (rule 4).
• Regression commits precede their fixes (rule 5).
• Test expectation changes are documented above.

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