refactor(insurance): return typed InsuranceError instead of panicking

[yunus-dev-codecrafter]

closes #712

Summary

Refactored the insurance contract to return typed InsuranceError instead of panicking, aligning it with the workspace error-handling convention used by other contracts (RemittanceSplitError, SavingsGoalError, BillPaymentsError).

Changes

• Converted all public functions to return Result<T, InsuranceError>:

  • init(), create_policy(), pay_premium(), batch_pay_premiums()
  • set_external_ref(), deactivate_policy(), get_active_policies()
  • get_policy(), get_total_monthly_premium()

• Converted internal helpers to return Result:

  • require_initialized(), get_owner(), load_policy(), validate_ext_ref()

• Mapped all panics to existing InsuranceError variants:

  • Empty/too-long name → InvalidName
  • Bad premium → InvalidPremium
  • Bad coverage → InvalidCoverageAmount
  • Ratio guard → UnsupportedCombination
  • Bad external ref → InvalidExternalRef
  • Cap hit → MaxPoliciesReached
  • Missing policy → PolicyNotFound
  • Inactive → PolicyInactive
  • Non-owner → Unauthorized
  • Uninitialized → NotInitialized
  • Already initialized → AlreadyInitialized

• Added Rust doc comments documenting which errors each public function can return

• Updated tests to handle Result types with .unwrap()

Why This Matters

Panics abort transactions with opaque host errors, preventing callers (CLI, indexer, orchestrator cross-contract flows) from distinguishing recoverable validation failures from real faults. Typed errors are the contract's public ABI for failure — the insurance contract was the only one missing this.

Acceptance Criteria

• ✅ No panic!, unwrap(), or expect() in non-test insurance code
• ✅ Every InsuranceError variant is reachable
• ✅ ABI discriminants unchanged (1..11)
• ✅ All existing tests updated to handle Result types

[Baskarayelu]

yes, swapping the panics in create_policy/pay_premium for typed InsuranceError is exactly the hardening insurance needed. clean diff. merging.