Implement Integration Test for Continue Streak Flow

[popsman01]

Description
Write an end-to-end integration test covering win, continue, second win, and final cash out.

Motivation
Tests are the primary safety net for the Tossd contract. Without coverage for this area, regressions can go undetected and break game fairness or fund safety guarantees.

Requirements and context

• Must be secure, tested, and documented
• Should be efficient and easy to review
• Primary files: Tossd-contract/src/lib.rs

Proposed Solution
Complete the following in a focused PR off feature/integration-continue-flow:

• Write full continue streak flow test
• Verify multiplier progression and streak persistence
• Document expected sequence behavior

Technical Implementation

Files to modify:

• coinflip-contract/src/lib.rs

Implementation steps:

• Write full continue streak flow test
• Verify multiplier progression and streak persistence
• Document expected sequence behavior

Branch: feature/integration-continue-flow

Example commit message

test: add integration coverage for continue streak flow

Acceptance Criteria

• Run tests
• Cover edge cases
• Include test output and flow notes

Guidelines

• Assignment required before starting
• PR description must include: Closes #152

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[kryputh]

@popsman01 @daatsuka @unixfundz @Macnelson9 Please assign