test: expand CSE machine test coverage to 61 tests

[Akshay-2007-1]

Summary

Expands test coverage for the three CSE machine packages from 8 tests to 61.

• common (1 → 22 tests): constants values, structured-clone safety for all snapshot shapes, CseSnapshotMessage invariants, optional fields on CseSnapshot, CseSerializedValue, CseSerializedInstruction, CseSerializedEnvFrame, and CseSerializedBinding
• runner (2 → 13 tests): plugin identity, constructor error handling, sendSnapshots message shape, totalSteps accuracy, empty/large batches, field preservation
• web (5 → 26 tests): plugin identity, constructor error handling, valid message delivery, ordering, field preservation, invalid/malformed message rejection (wrong type, null, undefined, non-array snapshots, totalSteps mismatch, missing fields), re-export contract

Test plan

• yarn test passes with all 61 tests green
• No source files modified

[changeset-bot]

⚠️ No Changeset found

Latest commit: d39e489

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[gemini-code-assist]

Code Review

This pull request significantly expands the test coverage for the CSE machine across the common, runner, and web packages, introducing structured test suites for constants, structured-clone safety, and message validation. The review feedback focuses on code quality improvements in the test files: extracting a duplicated array to a constant in common.test.ts to avoid unnecessary allocations, and refactoring the makeChannel helper in runner.test.ts to return a strongly-typed mock, which eliminates the need for multiple redundant type assertions throughout the file.

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