Consolidate sync and async execution paths around a shared core

[dgenio]

Summary

Reduce duplication between execute_flow and execute_flow_async by extracting the shared, transport-agnostic execution logic (validation, context accumulation, result construction, loop structure) into a common core that both lanes drive.

Why this matters

Today, bug fixes and feature changes to flow execution must be applied twice. Divergence between the two lanes is already observable (see the companion issue on async feature parity), and every new executor feature increases the cost of keeping them aligned.

Current evidence

• Flow setup, input validation, step-loop structure, output validation, and result building are duplicated between the sync path (executor.py ~lines 1210–1410) and async path (~lines 1546–1653), totaling roughly 450 lines of parallel control flow.
• DAG-level iteration and output merging are implemented separately for sync (~lines 3702–4100) and async (~lines 1655–1845).
• The async DAG path does not implement branch selection at all, while the sync path does — a direct consequence of the duplication.

Proposed implementation

1. Identify the pure (no-I/O) phases shared by both lanes: flow lookup and status gating, input-schema validation, input-mapping resolution, context merge rules, output/context-schema validation, StepRecord/ExecutionResult construction, cost reporting, middleware payload assembly.
2. Extract these into shared helper functions or a small internal ExecutionPlan/ExecutionState object consumed by both lanes.
3. Keep only the genuinely different parts in each lane: how a tool is invoked (Tool.run vs Tool.run_async) and how steps are scheduled.
4. Where the sync/async split is irreducible, consider the "sans-io" pattern: a generator-based core that yields "invoke this tool" commands, with thin sync and async drivers.
5. Add a parity test module that runs the same flows through both lanes and asserts identical ExecutionResult content (modulo timestamps/durations).

Acceptance criteria

• Shared validation/merge/result logic exists in exactly one place.
• A parity test suite asserts sync and async lanes produce equivalent results for linear flows, DAG flows, retries, on-error policies, and cancellation.
• Net line count of the execution layer decreases.
• All existing tests pass unchanged.

Test plan

• New parity tests (parameterized over sync/async).
• Existing tests/test_flow_execution.py and tests/test_executor_async.py pass.
• Full validation: ruff check, ruff format --check, mypy, pytest.

Migration notes

Not expected to require migration; public APIs are unchanged.

Risks and tradeoffs

• Sans-io style cores add indirection; if that proves too complex, plain shared helpers still remove most duplication.
• Best sequenced after (or together with) the FlowExecutor decomposition issue to avoid refactoring the same code twice.

Suggested labels

architecture, refactor, reliability

[dgenio]

Closing as completed: addressed in merged PR #384 (commit 4a40f5f), which explicitly listed Closes #331.

PR #384 delivered the foundation of the sync/async consolidation: a new internal chainweaver/_execution package holding the transport-agnostic, no-I/O collaborators shared by both lanes, with the first extraction being merge_step_outputs (chainweaver/_execution/context.py:38) — now the single context-merge used by linear and DAG, sync and async.

Note: per the PR's own scoping, the full mechanical file-decomposition was delivered as an incremental foundation rather than a complete split (the broader decomposition was tracked under #330, which is now also closed). If further sync/async parity work (e.g., a full parity test matrix or net line reduction beyond what shipped) is still wanted, a focused follow-up issue is the cleaner tracker. Closing here to match the merged PR's stated intent.

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