fix: stream validation and test coverage

[overprodigy]

Summary

This PR addresses 4 stream-related issues: event filtering, test coverage, and financial invariants.

Changes

#364 - Stream Events Endpoint

• Add FEE_COLLECTED to valid event types in stream events filter
• Endpoint already has pagination (limit/offset), total count, and hasMore flag
• Endpoint supports eventType filtering with all 8 event types

#365 - Pause/Resume Edge Cases

• Add 5 new test cases covering pause/resume scenarios:
  • Cancel while paused keeps stream inactive
  • Top-up while paused increases deposited amount
  • Withdraw after long runtime is bounded
  • Withdraw then resume preserves state
  • Multiple pause/resume cycles maintain active state

#366 - Overflow Protection

• Overflow protection already implemented in contract (checked_mul with saturation)
• Backend service uses saturatingMulI128 to clamp values at i128 limits
• Both implementations cap claimable at remaining balance on overflow

#367 - Fuzz Tests

• Add 5 property-based fuzz tests covering financial invariants:
  • Withdrawn amount never exceeds deposited amount
  • Claimable amount never exceeds remaining balance
  • Cancel early refunds correctly
  • Pause/resume cycles maintain active state
  • Large amounts don't overflow claimable calculation

Testing

• All 78 contract tests pass
• New tests validate edge cases and invariants
• Tests use deterministic pseudorandom seed for reproducibility

Closes #364
Closes #365
Closes #366
Closes #367