fix(workflows): require a cases block on switch steps

[jawwad-ali]

Problem

SwitchStep.validate requires expression and type-checks cases/default, but never checks that cases is present. It is the only control-flow step whose branch payload is optional:

step	required branch payload
if	then
fan-out	items and step
fan-in	non-empty wait_for
gate	message
while / do-while	condition
switch	— none —

Reproduction on current main (bf88c9f)

sibling contrast:
  if   missing then : ["If step 'x' is missing 'then' field."]
  fanout missing all: ["Fan-out step 'y' is missing 'items' field.",
                       "Fan-out step 'y' is missing 'step' field ..."]

switch:
  typo `case:` not `cases:` -> []          <-- validates clean
  no cases key at all       -> []          <-- validates clean

  execute -> status=completed matched='__default__' next=[]

So a case:/cases: typo passes specify workflow validate with zero errors, then at run time reports COMPLETED with matched_case: "__default__" — a default it does not even declare — having dispatched nothing. The whole run reports success while the branch never executed.

That is precisely the "silent empty result + COMPLETED" wiring bug the fan-in wait_for guard and the engine's fan-in check exist to prevent.

Fix

One presence check, mirroring the sibling steps' wording.

Narrow. An explicitly declared but empty cases: {} is still a declaration and stays valid — pinned by test_validate_accepts_an_empty_cases_mapping. Only configs that never declared the block at all become errors, and those already do nothing at run time.

Verification

• Fail-before / pass-after on a targeted scope (-k "Switch or validate or Validat"): 1 failed → 202 passed.
• tests/workflows → 171 passed, 10 failed, matching the clean-main baseline exactly.
• uvx ruff@0.15.0 check src tests → clean
• Tests go into the existing TestSwitchStep class, next to test_validate_missing_expression.

A note on the full-file run. tests/test_workflows.py is intermittently flaky on Windows — a rotating handful of tests (TestFanOutConcurrency, TestContextWorkflowDir, TestContinueOnError, TestWorkflowRunExitCodes) fail on different runs and pass in isolation, from os.replace PermissionError. I confirmed this is unrelated to this change: test_sequential_and_concurrent_agree fails 2 of 4 runs on clean main and passes 4/4 with this patch applied, and the two other flagged tests pass in isolation. Hence the targeted verification above.

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Written with assistance from Claude Code. Bug found, reproduced, and verified by me on current main.