test: add proptest coverage for task-registry and reward-engine (#86) - #90

Summary

eco-token had proptest coverage for mint/transfer/burn (#41); task-registry
and reward-engine had none. This adds property-based fuzz tests for both,
following eco-token's existing pattern (ProptestConfig::with_cases(20)).

task-registry (3 properties)

• proptest_create_task_validation — create_task succeeds iff
  reward_amount > 0 && max_completions > 0 && expires_at > now, over
  randomized inputs including boundary/negative cases.
• proptest_expiry_boundary — complete_task succeeds iff the probe
  timestamp is <= expires_at, exercising the strict-less expiry
  semantics right around the boundary.
• proptest_completion_count_invariant — asserts completions <= max_completions holds after every call in a randomized sequence, and
  that status flips to Completed exactly when the cap is hit.

reward-engine (3 properties)

• proptest_reward_range_enforced — approve_proof succeeds iff
  reward_amount is within the configured [min_reward, max_reward]
  and within the task's declared budget.
• proptest_total_paid_accumulation — total_paid() equals the running
  sum of approved rewards across a randomized sequence of approvals.
• proptest_cooldown_boundary — a second approval for the same user
  succeeds iff elapsed ledgers since the last reward are >= cooldown,
  covering the saturating_sub boundary from both sides.

Other changes

• Added proptest = "=1.6.0" to [dev-dependencies] in both
  task-registry/Cargo.toml and reward-engine/Cargo.toml (previously
  only eco-token had it).

Verification note

I was not able to run cargo test myself against a full Soroban
toolchain — I've cross-checked the function signatures, imports, and
types used here against the actual source (create_task,
complete_task, Ledger trait usage, etc.), but this hasn't been
compiled end-to-end. Flagging that explicitly so review/CI catches
anything I couldn't. Happy to fix quickly if CI turns up an issue.

Out of scope

• Adding new notification types / production logic changes
• Integration fuzz, mutation testing

Closes #86