test(packaging): pin native manifest field refusals

[harshitethic]

Summary

Follow up on the field-validation refusals explicitly left out of #408 by adding focused coverage for the native-overlay manifest scalar/path validators.

The tests pin refusal behavior for:

• object and exact-field validation;
• integer, positive and non-negative integer constraints;
• printable strings and dotted Python identifiers;
• POSIX-only relative paths and root-escape rejection;
• canonical lowercase SHA-256 values.

A final acceptance case records the canonical values each helper accepts. This is deliberately separate from the real-overlay parse-stage coverage in #417: these are small boundary validators and do not need a compiler-backed overlay build for every scalar case.

Related: #408.

Validation

The change is test-only and network-free. GitHub Actions is the authoritative full-suite validation for this branch.

AI assistance disclosure: AI assistance was used to inspect the deferred scope in #408, prepare the focused parametrized tests, and review the resulting change.

[github-actions]

👋 Hi @harshitethic — thanks for the contribution! To keep starter issues available
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You already have #415 open, so this one is being closed automatically.
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