fix(soroban): mock invokeContract returns real results; regression tests for #581/#582

[jajafwangshak86-ops]

Closes #573
Closes #581
Closes #582

What changed

#573 — invokeContract returns "Not implemented"
Root cause was the mock client: soroban.invokeContract always resolved with data: null, and the Soroban screen renders a null result as "Not implemented". No component rendering was at fault. Fixed at the source in src/lib/mock-client.ts — the mock now resolves with a deterministic, non-null result ({ success: true, result: {...}, txHash }), so the panel shows a real invocation result.

#581 — unsafe casts on submit buttons
Verified the offending code is already gone from main: both SorobanPanel.tsx and TransactionPanel.tsx open their forms natively via type="submit" buttons linked through the form attribute. This PR pins that behaviour with regression tests so it can't silently regress:

• SorobanPanel.test.tsx — Invoke button is type="submit", points at the form id, and clicking it reaches the form's onSubmit and a real invocation.
• TransactionPanel.test.tsx — Send Payment button is type="submit" tied to the form.

#582 — ContractEventFeed stale-closure polling
Verified the fix is already in main: load is memoized with useCallback([contractId, limit, filterTypes, client]) and the polling interval effect restarts when contractId changes. Added polling contractId switching (#582) regression tests that switch contractId mid-poll (with fake timers) and assert every poll after the switch hits the NEW contract id, plus a Live-toggle pause/resume test.

Test harness fix

SorobanPanel/TransactionPanel tests mocked useSorokit without a client, so invocations never reached the mocked API (the pre-existing suite failures). The mocks now expose the client the panels actually read, bringing the panels' success-state tests back to green.

Verification

• npm run typecheck passes.
• eslint on changed files passes.
• Changed files: 117 passed / 6 failed (the 6 remaining are pre-existing, unrelated to this PR: ContractEventFeed JSON-export tests query /export json/i against an aria-label of "Export {n} events as JSON", and 3 TransactionPanel tests that override the context mock with balance arrays).

Commits: 1

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