[Critical] Add Stream module unit tests

[randomm]

Problem

packages/opencode/src/core/stream.ts (484 lines) has no dedicated tests.

Required Tests

• Doom loop detection (same tool called 3x with identical input)
• Tool result injection
• Error recovery
• Context overflow detection
• Thinking block extraction
• Abort handling during cleanup

Files

• Create: packages/opencode/test/core/stream.test.ts

Coverage Target

80%+ line coverage for stream.ts

Branch

claude/simplify-agentic-loop-2jRSP

[randomm]

Stream tests were added in PR #35 (commit 5e8aebd) as part of the task slot fix work. The test file exists at packages/opencode/test/core/stream.test.ts.