Fix Switch resume from serialized scheduled state

[Copilot]

Purpose

Prevent Switch from throwing when a workflow resumes after ScheduledActivityIds has round-tripped through JSON serialization.

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Scope

Select one primary concern:

• Bug fix (behavior change)
• Refactor (no behavior change)
• Documentation update
• Formatting / code cleanup
• Dependency / build update
• New feature

If this PR includes multiple unrelated concerns, please split it before requesting review.

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Description

Problem

Switch stored scheduled child IDs as a HashSet<string>, but serialized workflow state restores that value as List<string>. The completion callback then read the value back as HashSet<string>, causing an InvalidCastException on resume.

Solution

• Read ScheduledActivityIds as ICollection<string> so both the original HashSet<string> and restored List<string> are accepted.
• Add a regression test covering child completion with list-restored scheduled activity IDs.

var scheduledActivityIds = context.TargetContext.GetProperty<ICollection<string>>("ScheduledActivityIds");

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Verification

Steps:

1. Run the targeted SwitchTests.
2. Verify the restored-list regression test completes the Switch activity.
3. Verify existing switch scheduling behavior remains unchanged.

Expected outcome:

Switch completes normally after resumed child completion when ScheduledActivityIds is restored as List<string>.

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Screenshots / Recordings (if applicable)

N/A — no UI changes.

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Commit Convention

We recommend using conventional commit prefixes:

• fix: – Bug fixes (behavior change)
• feat: – New features
• refactor: – Code changes without behavior change
• docs: – Documentation updates
• chore: – Maintenance, tooling, or dependency updates
• test: – Test additions or modifications

Clear commit messages make reviews easier and history more meaningful.

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Checklist

• The PR is focused on a single concern
• Commit messages follow the recommended convention
• Tests added or updated (if applicable)
• Documentation updated (if applicable)
• No unrelated cleanup included
• All tests pass