Clarify nested defer pattern and fix incorrect log message in fdw.go

[e-gineer]

Description

The goFdwBeginForeignScan() function in fdw.go has nested defer statements for panic recovery, but the code lacks comments explaining why this pattern is used. Additionally, the outer defer has an incorrect log message.

Current Code Issues

1. Incorrect log message

The outer defer logs goFdwExplainForeignScan but we're in goFdwBeginForeignScan:

func goFdwBeginForeignScan(node *C.ForeignScanState, eflags C.int) {
    defer func() {
        if r := recover(); r != nil {
            log.Printf("[WARN] goFdwExplainForeignScan failed with panic: %v", r)  // WRONG!
            // ...
        }
    }()

2. Missing documentation

The nested defer pattern is intentional but not documented:

defer func() { /* outer recover */ }()
// ... early init code ...
defer func() { /* inner recover */ }()
// ... main processing ...

Proposed Fix

1. Fix the log message to say goFdwBeginForeignScan
2. Add comments explaining why nested defers exist:
   • Outer: catches panics during early initialization
   • Inner: catches panics during main scan processing

Impact

• Risk: None - Documentation only
• Functionality: No behavioral change
• Benefit: Easier maintenance and debugging