🧹 Refactor large test function into smaller focused tests

[seonghobae]

🎯 What: Extracted FakeQueue, FakeProcess, and FakeContext into a helper function and split a monolithic test function test_stem_separation_process_helper_maps_worker_results into four focused tests.

💡 Why: Splitting long functions improves readability and testability. The original test had multiple scenarios bundled together (ok envelope, ok file envelope, invalid file payloads, and error envelopes). Separating them makes the test failures easier to identify and reading the test suites easier.

✅ Verification:

• Extracted the Fake* classes to a helper and tested to make sure it worked.
• Replaced the target test with the 4 split ones and verified with pytest that they all pass.
• Formatted the files with ruff and made sure they pass static type checks.

✨ Result: The monolithic test has been split into four precise, isolated test functions resulting in increased modularity and readable code.

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PR created automatically by Jules for task 8262543579606685017 started by @seonghobae

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[opencode-agent]

OpenCode Review Overview

• Head SHA: 4f03b1b401e3e07e82e456291b034dac462bb5aa
• Workflow run: 27909923132
• Workflow attempt: 1
• Gate result: OPENCODE_REVIEW_UNAVAILABLE (approval step)

OpenCode Agent did not produce a valid review payload after all current-head GitHub Checks completed.

• Result: OPENCODE_REVIEW_UNAVAILABLE
• Reason: OpenCode review attempts did not complete or did not return a valid control block.
• OpenCode outcomes: primary=failed, fallback=failed, second_fallback=failed

OpenCode runtime evidence:

• primary model call failed: Request body too large for gpt-5 model. Max size: 4000 tokens.
• Head SHA: 4f03b1b401e3e07e82e456291b034dac462bb5aa
• Workflow run: 27909923132
• Workflow attempt: 1

No blocking review was submitted because this is an agent/runtime failure, not a source-backed code finding.