fix(stepfunctions): idempotent StartExecution, history detail keys, Choice/Pass field handling

[vieiralucas]

Fixes a cluster of confirmed Step Functions correctness bugs surfaced by the 2026-06-28 bug hunt.

Bugs fixed

1. StartExecution was never idempotent (HIGH). A name collision always returned 400 ExecutionAlreadyExists without comparing input. AWS (STANDARD) returns the existing executionArn with HTTP 200 when the name AND input match; only a different input returns 400. Now compares the stored input on collision and returns the existing execution when identical.

2. GetExecutionHistory used the wrong details key for every State Entered/Exited event (HIGH). The key was derived by lowercasing the first char, yielding taskStateEnteredEventDetails / passStateExitedEventDetails etc. AWS collapses all *StateEntered types into stateEnteredEventDetails and all *StateExited into stateExitedEventDetails. SDK deserializers were returning null name/input/output across the whole history. Now mapped to the collapsed keys.

3. Choice ignored OutputPath; Pass ignored Parameters (MEDIUM). A Choice state forwarded raw input instead of applying its OutputPath; a Pass state passed input through instead of evaluating its Parameters template. Both now apply the same OutputPath/Parameters evaluation the other states use.

Tests

E2E coverage in crates/fakecloud-e2e/tests/stepfunctions.rs for idempotent StartExecution (same-input 200 / different-input 400), collapsed history detail keys, and Choice OutputPath + Pass Parameters.

Local: build, fmt --check, clippy --all-targets -D warnings, 192 unit tests, e2e compile all green.

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Summary by cubic

Fixes Step Functions idempotency, history event detail keys, and Choice/Pass field handling to match AWS behavior. Adds explicit E2E tests for idempotent StartExecution.

• Bug Fixes
  • StartExecution idempotent for STANDARD: same name + input returns existing executionArn (200) via structural JSON compare; different input returns 400 ExecutionAlreadyExists. EXPRESS unchanged.
  • GetExecutionHistory uses collapsed detail keys: all *StateEntered -> stateEnteredEventDetails, all *StateExited -> stateExitedEventDetails, restoring SDK name/input/output.
  • State evaluation: Choice applies OutputPath; Pass evaluates Parameters to build the payload before Result/ResultPath and OutputPath.

^{Written for commit 3be0f4e. Summary will update on new commits.}