test(unikontainers): add unit tests for pure functions

[mdryaan]

Description

Adds unit tests for three sets of pure functions in pkg/unikontainers that have real conditional logic but no test coverage.
TestMapRootfsPropagationFlag — mount_test.go Covers all 8 valid propagation flag names (shared, rshared, slave, rslave, private, rprivate, unbindable, runbindable) asserting the exact unix flag value returned, plus one subtest for an unknown value asserting a non-nil error.

TestChooseTmpfsSize — shared_fs_test.go Covers the 9pfs constant path, two virtiofs cases with typical memory (256 MB, 512 MB) verifying the MiB-to-MB conversion and 1 MiB overhead, and the zero-memory edge case.

TestInt32msgSerialize / TestBytemsgSerialize — ipc_message_test.go Covers int32msg serialization (length, type, and value fields at thecorrect byte offsets) and bytemsg serialization (aligned buffer length, header fields, value bytes). Includes a subtest for the oversized-value panic path in bytemsg that asserts a netlinkError is raised when Len() exceeds math.MaxUint16.

Related issues

• Refs Add more unit tests #96

How was this tested?

go test -count=1 ./pkg/unikontainers/... -v
-run "TestMapRootfsPropagationFlag|TestChooseTmpfsSize|TestInt32msgSerialize|TestBytemsgSerialize"
All 16 subtests pass. Linter passes (make lint, 0 issues).
Build passes (make).

LLM usage

Claude Sonnet 4.6 was used to assist in identifying untested functions and understanding the codebase structure. All code was manually reviewed and understood before submission.

Checklist

• I have read the contribution guide.
• The linter passes locally (make lint).
• The e2e tests of at least one tool pass locally (make test_ctr, make test_nerdctl, make test_docker, make test_crictl).
• If LLMs were used: I have read the llm policy.

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