Virtual Context bug on parent id and FAIL checkpoint emitting

[yaythomas]

Description of changes:

Fixes two functional bugs in virtual child contexts and refactors the supporting mechanism.

#363 — run_in_child_context(is_virtual=True) sends invalid parent_id to the backend. On main, inner operations inside a virtual run_in_child_context stamp the branch's own operation id as their parent_id. The branch itself writes no START/SUCCEED under is_virtual=True, so that id does not exist in the execution history. The backend correctly rejects the checkpoint with InvalidParameterValueException(CHECKPOINT_INVALID_PARENT_OPERATION_ID), and the user's execution fails at the first inner checkpoint. Nesting virtual scopes (and FLAT map/parallel inside a virtual scope) compounds the failure. After this change, inner operations stamp the enclosing non-virtual ancestor, which is a real checkpointed parent.

#362 — virtual branches wrote a FAIL checkpoint on exceptions. On main, an exception inside a virtual branch caused ChildOperationExecutor.execute to unconditionally write a FAIL checkpoint for the branch — producing a phantom FAILED CONTEXT entry in the execution history with no matching START. On the FLAT map/parallel path this was cosmetic (the FAIL's parent id was the valid map/parallel op, the backend accepts a CONTEXT FAIL without a prior START); on the run_in_child_context(is_virtual=True) path it compounded #363 (the FAIL's parent id was the same orphan branch id, which the backend rejects). Either way it's incorrect: virtual branches should emit no lifecycle entries at all. After this change, virtual branches emit no lifecycle checkpoints; failures still propagate to the concurrency executor's BatchResult (or to the caller of run_in_child_context), and completion-tolerance logic still applies.

Mechanism changes (applied alongside the fixes):

• Single source of truth for the virtual-vs-real decision. create_child_context computes two fields (_parent_id, _step_id_prefix) at construction; no per-operation-method knowledge is required. New operations just read self._parent_id and work correctly under both modes.
• Field names match their roles. _parent_id is "the id my inner operations stamp as their parent"; _step_id_prefix is "how I prefix step ids". Each field has one job.
• is_virtual is encapsulated in the context as a cached property. Callers opt in with create_child_context(..., is_virtual=True); the property makes the state inspectable.
• Nested virtual-in-virtual matches the JS reference. A virtual child of a virtual parent inherits its parent's reporting ancestor, so chained FLAT layers collapse to the outermost non-virtual context without dangling parent-id references.
• ChildConfig.is_virtual drives lifecycle-checkpoint suppression in ChildOperationExecutor (START, SUCCEED, FAIL) and, via run_in_child_context, the child context's own virtual-ness. The field remains public, matching the JS SDK's ChildConfig.virtualContext.
• Fewer parameters threaded through. operation_identifier is gone from ConcurrentExecutor, MapExecutor, and ParallelExecutor constructors and from_items/from_callables; the concurrency layer no longer needs an OperationIdentifier to figure out the reporting parent.
• Tests exercise the invariants directly with a real DurableContext and assert wire-format decisions, including nested-virtual-in-virtual coverage.

Impact:

• Users can call ctx.run_in_child_context(config=ChildConfig(is_virtual=True)) without their execution being rejected at checkpoint time.
• FLAT map/parallel no longer emits phantom FAILED CONTEXT entries on branch failure.
• Python SDK virtual-context semantics now match the JS reference SDK exactly (same parent-id propagation rule; nested virtuals collapse identically).
• Cost: eliminates billable FAILED CONTEXT entries that were previously emitted for failed virtual branches.

Also includes minor CONTRIBUTING.md improvement (hatch venv symlink tip for editors that mangle paths with spaces).

Issue #, if available:

Fixes #362, Fixes #363

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[zhongkechen]

Let's remove is_virtual from ChildConfig. It's a mistake to expose this field to users. This is supposed to be used only by concurrency operations internally