Expand contracts CI to test `group_treasury` and `proposals`

[codebestia]

The current .github/workflows/contracts-ci.yml only tests token_transfer. Add cargo test -p group_treasury and cargo test -p proposals steps after the existing test step. Update the WASM build step to build all three packages: token_transfer, group_treasury, and proposals.

Acceptance criteria:

• All three cargo test -p <pkg> steps appear in the workflow
• A failing test in group_treasury or proposals fails the CI run
• All three contracts produce release WASM artefacts

[DeveloperEmmy]

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