[Testing] Add contract fuzz tests for create_stream with edge-case amounts

[Godbrand0]

Resolves #425

Changes

• Added proptest for fuzzing tests on stream edge cases in Soroban
• Tested that amount = 0 correctly handles intercept errors without panicking.
• Validated no overflow errors against i128::MAX volume deposits.
• Verified stream handling matches minimum constraints under 1 ledger increments.
• Confirmed exact truncating and refund bounds on remainders failing divisible metrics. Updated corresponding module documentation outlining exactly when stream fractions distribute.

[ogazboiz]

hey, main was failing CI from broken auth imports + frontend parse errors. fixed and pushed to main now. please rebase to pick up the fixes:

git fetch upstream
git rebase upstream/main
git push --force-with-lease

if there's a conflict, resolve it locally and we'll review once CI is green.

[ogazboiz]

hey, main CI is fully green now. there were several real TypeScript and test issues from #444's merge that have all been fixed. please rebase one more time:

git fetch upstream
git rebase upstream/main
git push --force-with-lease

your PR's diff should be much cleaner after this rebase since main now has the canonical versions of cancel.ts, sorobanService.ts, and the withdraw test.

[ogazboiz]

closing for inactivity — this has been open ~4 weeks with no contributor response after the last review. main has moved significantly since (#499 contract reconciliation, #706 auth-mock fix, ~150 new issues filed). if you want to continue this work, open a fresh PR rebased on current main. happy to review it then.

join us on Telegram if you want to coordinate: https://t.me/+DOylgFv1jyJlNzM0