Refactor contract Tests to use property-based and invariant testing

[Smartdevs17]

Description

Contract tests are example-based missing edge cases. Refactor to property-based testing for comprehensive coverage of contract invariants.

Acceptance Criteria

• Property definitions for all critical invariants
• proptest integration with seed corpora
• State machine testing for multi-step scenarios
• Invariant test suite for fuzz testing
• CI integration with shrinking
• Test coverage report comparison

Technical Scope

• contracts/tests/invariants/
• contracts/subscription/tests/
• contracts/types/src/
• Edge: test timeout for complex properties, state explosion in state machine tests

[TUPM96]

Implemented in #441

What changed:

• Added package-level Soroban property/invariant tests for subscription flows using the real storage contract and generated clients.
• Covered plan count consistency, charge accounting, lifecycle transitions, refund bounds, and user/plan subscriber invariants.
• Updated invariant CI to run the registered property_invariants test target with CI-controlled PROPTEST_CASES / shrink settings.

Verified locally:

• cargo fmt --check
• PROPTEST_CASES=16 PROPTEST_MAX_SHRINK_ITERS=1000 cargo test -p subtrackr-subscription --test property_invariants -- --nocapture
• cargo test --workspace --lib --verbose
• git diff --check

[temi-Dee]

@temi-Dee has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

hi maintainer i can work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @temi-Dee to this issue.