test(share): add full test coverage for share token contract

[DeborahOlaboye]

Summary

Closes #578

The share token contract had zero tests despite being the voting-power primitive for governance. This PR adds a complete test suite across three layers:

• contracts/share/tests/lib.rs — 12 integration tests covering all checklist gaps: allowance overwrite, zero-approve clearing, transfer_from with insufficient balance, transfer_from to self, burn auth enforcement, sender auth on transfer, and i128 large-value overflow safety
• contracts/share/tests/fuzz_tests.rs — 5 property-based tests (proptest, 50 cases each): total_supply == sum of balances, balance >= 0 invariant, allowance-never-underflows, transfer is net-zero on supply, burn reduces supply exactly
• contracts/share/Cargo.toml — added rlib crate-type, proptest = "1.4" dev-dep, and [[test]] entries for both test files
• .github/workflows/ci.yml — added share to the rust-contracts matrix (fmt + clippy + build + test on every PR)

Test plan

• cargo test -p share passes locally — 33 tests total (16 inline + 12 integration + 5 fuzz)
• All issue checklist items covered
• CI matrix updated to run share contract on every PR

[sanmipaul]

@DeborahOlaboye Great work on this, the proptest invariants are well-chosen, the doc-comments explaining why each property matters are exactly right, and the CI matrix addition is clean. A few things before merge:

Must fix

test_burn_requires_admin_auth has no tokens minted before attempting the burn.
holder has zero balance, so the test may be failing on "insufficient balance" rather than "unauthorized" — it passes for the wrong reason. Mint tokens to holder first so the test actually isolates the auth check:

client.mint(&holder, &100i128);
let result = client.try_burn(&holder, &100i128);
assert!(result.is_err());