Add comprehensive tests for vesting_escrow contract edge cases

[Wilfred007]

What

Only one test covers the entire vesting contract. Missing tests for: double init, claim before cliff, claim when fully claimed, clawback by non-admin, zero-amount init, duration < cliff, multiple partial claims.

Why

The single test doesn't verify error paths. The panic! error paths are completely untested.

Scope

In scope: Tests for all error variants, boundary conditions, authorization checks
Out of scope: Fuzzing, property tests

Acceptance Criteria

• Test: double initialization rejected
• Test: claim before cliff fails
• Test: unauthorized clawback rejected
• Test: zero-amount init rejected
• Test: multiple partial claims work

Technical Context

• contracts/vesting_escrow/src/test.rs — only 1 test (114 lines)
• contracts/bulk_payment/src/test.rs — good test pattern reference