Python: Surface switch-case condition errors instead of routing to the default branch

[Ruiqi Wang (RachelWanggg)]

Motivation & Context

SwitchCaseEdgeGroup wrapped every case predicate in a bare except Exception that only logged a
warning. A predicate that raised was treated exactly like one that returned False, so the message
fell through to the default branch and the workflow completed successfully. A typo or a missing
attribute in a routing predicate became silently wrong routing instead of a visible failure.

This contradicted the rest of the routing layer:

• Edge.should_route documents the opposite policy for the same concept — "Any exception raised by
  the callable is deliberately allowed to surface to the caller to avoid masking logic bugs."
• FanOutEdgeRunner.send_message already handles a selection function that raises: it marks the
  edge-group span EDGE_GROUP_DELIVERY_STATUS = EXCEPTION and re-raises. The inner catch made that
  path unreachable for switch-case groups, so these failures were absent from telemetry as well.
• The .NET predicate path (SwitchBuilder.cs / WorkflowBuilder.CreateConditionFunc) has no
  equivalent catch.

The block was marked # pragma: no cover, so nothing exercised it.

Description & Review Guide

• What are the major changes?

  1. _edge.py — SwitchCaseEdgeGroup.selection_func no longer catches predicate errors; they
     propagate to the caller, matching Edge.should_route.
  2. _edge_runner.py — FanOutEdgeRunner.send_message drops a message that no target can handle
     before running the selection function, marking the span DROPPED_TYPE_MISMATCH and
     returning False. This mirrors SingleEdgeRunner, which checks _can_handle before
     evaluating Edge.should_route.
  3. Three tests in test_edge.py covering both behaviours.

• Why change 2 is part of this PR. SingleEdgeRunner only ever evaluates a predicate for a
  message the target could actually receive; FanOutEdgeRunner evaluated the selection function
  first. The existing test_switch_case_edge_group_send_message_with_invalid_data passes a str
  into a lambda x: x.data < 0 predicate and depended on the swallow to reach its
  success is False assertion. Without the ordering fix, change 1 alone would turn that
  undeliverable-message case into a raised AttributeError. With it, that test passes unchanged and
  genuine predicate errors still surface. I'm happy to split this into a separate PR if you'd
  rather keep the two concerns apart.

• What is the impact of these changes?

  • A broken switch-case predicate now fails loudly instead of misrouting to the default branch, and
    the failure is recorded on the edge-group span.
  • A selection function is no longer invoked for messages that no target can handle. Delivery
    outcome for those messages is unchanged (False); they are just dropped earlier.
  • No public API change.

• What do you want reviewers to focus on? Whether bundling the FanOutEdgeRunner ordering
  change with the swallow removal is the right call, or whether you'd prefer the existing
  ..._with_invalid_data test to be updated instead.

Verification

• uv run poe test -P core — full core + workflow suite passes (0 failures).
• uv run poe syntax -P core, uv run poe pyright -P core, uv run poe test-typing -P core
  (mypy, pyrefly, ty, zuban, pyright) — all pass.
• Each new test was mutation-checked: reverting change 1 fails the two condition-propagation tests;
  reverting change 2 fails the fan-out ordering test and the pre-existing
  ..._with_invalid_data test.

Related Issue

Fixes #8489

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change.

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[Ruiqi Wang (RachelWanggg)]

@microsoft-github-policy-service agree