test: add HbarAllowance unit coverage

[peter941221]

Summary

• add unit coverage for HbarAllowance constructor storage
• add protobuf round-trip coverage, including nil owner and nil spender cases
• add String() coverage for all four owner/spender nil combinations

Validation

• GOPROXY=https://goproxy.cn,direct go test ./sdk -tags="unit" -v -run TestUnitHbarAllowance -timeout 9999s
• GOPROXY=https://goproxy.cn,direct go build ./...
• GOPROXY=https://goproxy.cn,direct golangci-lint run

Notes

• go test ./sdk -tags="unit" -v -timeout 9999s currently fails in existing TestUnitConcurrentGetNodeReadmit with panic: failed to find a healthy working node.
• I reproduced that same failure on a clean origin/main mirror, so it is unrelated to this change.

Fixes: #1705

[lfdt-bot]

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[Dosik13]

Hi, can you give me more info about this:
go test ./sdk -tags="unit" -v -timeout 9999s currently fails in existing TestUnitConcurrentGetNodeReadmit with panic: failed to find a healthy working node.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
see 2 files with indirect coverage changes

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[peter941221]

Sure.

This was a local full-suite observation, not a failure introduced by this PR.

On my local Windows environment:

go version go1.25.7 windows/amd64

I can reproduce the panic by running the existing test directly:

go test ./sdk -tags="unit" -v -run TestUnitConcurrentGetNodeReadmit -count=10 -timeout 9999s

It fails with:

panic: failed to find a healthy working node
sdk/managed_network.go:185
sdk/network_unit_test.go:85

I also reproduced the same failure on a clean origin/main checkout at:

d91f3f2b1938fcd651b43bff9eeed7ca33fe5ec1

so it does not appear to be related to the new HbarAllowance test file.

The PR-specific test passes locally:

go test ./sdk -tags="unit" -v -run TestUnitHbarAllowance -timeout 9999s

Also, I see the GitHub Actions Unit and Integration Tests (unit) job is passing for this PR, so the hosted CI environment does not appear to hit the same local panic.

[Dosik13]

can you raise an issue about this - you could also work on it if you want

[peter941221]

Opened #1714 to track it separately. I'll keep this PR focused on the HbarAllowance unit coverage.

[Dosik13]

lgtm