fix(client): prevent panic when parsing events from multi-instruction txs

[alexchenai]

Summary

Fix panic in anchor-client when parsing events from transactions containing multiple top-level instructions.

Issue: #1941

Root Cause

The Execution struct's program() and pop() methods use assert!(!self.stack.is_empty()) which panics when the execution stack is depleted. This happens when a transaction contains multiple instructions - after the first instruction completes, its success log causes a pop() that empties the stack. The next iteration then calls program() on the empty stack, triggering the panic.

Fix

• Execution::program() now returns Option instead of panicking on empty stack
• Execution::pop() silently handles empty stack (no-op) instead of panicking
• parse_logs_response updated to compare against Option via as_deref()
• When the stack is empty, log lines are correctly processed as system logs

Tests

• test_parse_logs_response_multiple_instructions: Regression test with 3 interleaved top-level instructions
• test_execution_pop_empty_stack_no_panic: Unit test verifying graceful empty stack handling

[vercel]

@alexchenai is attempting to deploy a commit to the Solana Foundation Team on Vercel.

A member of the Team first needs to authorize it.

[jamie-osec]

This appears to simply ignore the errors rather than correctly handling the logic. Please refer to previous attempts to fix this: #1954

[alexchenai]

Thanks for the feedback and pointing to #1954. You were right — returning Option<String> and silently popping an empty stack was just ignoring the problem rather than fixing it.

Updated the fix to use pop_or_replace(next_program: Option<String>) which atomically transitions from one top-level instruction to the next, keeping the stack invariant intact. program() is back to returning String and will panic on a genuinely empty stack (which should now be unreachable).

The key insight from #1954: when we see Program X success followed by Program Y invoke [1], we should replace X with Y rather than pop-then-push, so the stack is never momentarily empty.