argo: implement fanout switch case support

[npow]

Summary

Follow-up to #3328 (Support fanout switch cases).

Implements Argo Workflows support for list-valued switch cases. The approach:

• Output: changes the switch-step Argo parameter from a scalar step name to a comma-separated list of chosen targets ("step1,step2" for fanout, "step" for single-target — no format change for existing flows).
• Conditions: updates all three when-condition sites in argo_workflows.py from ==step equality to a CEL membership test: split(',').exists(x, x == 'step'). This is backwards-compatible — a single-element list behaves identically to the old scalar comparison.
• Tests: adds switch_fanout core integration graph + hit/miss test cases; updates the test FlowFormatter to emit list-valued switch case values.

Test plan

• pytest test/unit/test_switch_fanout_cases.py — existing unit tests still pass
• pytest test/unit/test_graph_structure.py — graph structure tests still pass
• Argo core integration tests with the new switch_fanout graph (hit and miss cases)
• Existing switch core tests (switch_basic, switch_nested, etc.) still pass — the CEL split(',').exists(...) condition is equivalent to == for single-target cases

🤖 Generated with Claude Code

[greptile-apps]

Greptile Summary

Implements list-valued switch cases for Argo Workflows by serializing selected targets into one output parameter and using membership-based branch conditions.

• Removes compile-time and runtime rejection of switch fanout cases.
• Writes all selected targets as a comma-separated switch-step value.
• Updates DAG, recursive-step, and foreach conditions to test target membership.
• Adds formatter support and core integration coverage for single-target and fanout selections.

Confidence Score: 5/5

The PR appears safe to merge, with no concrete changed-code failure identified.

The new serialized switch output is consumed consistently at each routing site, and the reconverging fanout path retains both selected inputs while allowing the unselected path to be skipped without blocking completion.

Important Files Changed

Filename	Overview
metaflow/plugins/argo/argo_workflows.py	Removes fanout rejection and consistently changes all three switch-routing sites from scalar equality to membership checks.
metaflow/plugins/argo/argo_workflows_decorator.py	Serializes every selected switch target into the scalar Argo output parameter using a comma-delimited representation.
test/core/metaflow_test/formatter.py	Extends generated test flows to emit list-valued switch cases while retaining scalar-case formatting.
test/core/graphs/switch_fanout.json	Adds a graph covering a scalar case and a two-target fanout that reconverges at a join.
test/core/tests/switch_fanout_case.py	Verifies both the single-target path and the fanout path, including merged artifacts from both selected branches.

_{Reviews (1): Last reviewed commit: "argo: implement fanout switch case suppo..." | Re-trigger Greptile}

[saikonen]

this is leading to

Invalid 'when' expression '({{=(tasks['start'].status == 'Succeeded' ? tasks['start'].outputs.parameters['switch-step'].split(',').exists(x, x == 'branch_a') : false)}})': Invalid token: '{{=' (hint: try wrapping the affected expression in quotes ("))

in many test flows of mine.

[saikonen]

possibly similar issue with this expression.