test(chart): make default image assertions package portable

[weicao]

Problem

Packaging the chart with a release version changes .Chart.Version. Three StatefulSet tests hardcoded the source checkout version 0.0.1, so the exact release package from PR #7 produced 214 PASS / 3 FAIL even though explicit runtime image values rendered correctly.

Change

Set an explicit synthetic chart.version: 9.8.7-test fixture in the three tests that validate default image tags, and assert that tag. This locks the intended contract: the default image tag follows .Chart.Version regardless of source checkout or package metadata.

Production templates and values are unchanged.

Evidence

• base exact: d7255ec022952ab13a874ff928d2e32e6cfb50e2
• source helm unittest chart: 217/217 PASS
• helm package --version 0.35.1-b98b1ae5a-d7255ec0, extract, then helm unittest: 217/217 PASS
• helm lint chart: PASS
• git diff --check: PASS

This child PR must be reviewed before merging into PR #7. It does not change or block the existing explicit b98 fresh-runtime gate.

[weicao]

Focused review on exact head 7a8ab3968263801d33462c7a3fc069bcbf47c9c6, sole parent/base d7255ec022952ab13a874ff928d2e32e6cfb50e2: NO BLOCKER.

Independent checks:

• GitHub reports one modified file only: chart/tests/statefulset_test.yaml, +9/-3; no production template or values changes.
• The exact API patch matches the proposed pre-push diff.
• The three tests that depend on the default image tag now set chart.version: 9.8.7-test explicitly and assert the corresponding tag. This correctly tests that the default image tag follows .Chart.Version instead of relying on the source checkout's incidental 0.0.1 version.
• The custom-tag test remains independent.
• Applying the exact patch to the previously reviewed release-version package reproduces 217/217 PASS from the original packaged-chart 214 PASS / 3 FAIL state.

This is a test-portability approval only. It is not runtime PASS. If merged into PR #7, the candidate SHA/package SHA must be re-pinned before fresh runtime evidence is counted.