Fix middleware duplicated creation of transactions

[jptosso]

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[fzipi]

@jptosso Can you follow up on this one?

[fzipi]

@copilot Is this still valid?

[Copilot]

@copilot Is this still valid?

No, this PR is no longer valid. The previous change introduced compilation errors in http/middleware.go:

1. newTX was used with = (assignment) before being declared in scope
2. newTX in the else block was declared with := creating a local variable that was never used
3. newTX was undefined at the call site inside fn

The original pattern in the codebase (which is still present in main) is correct — declare newTX once with the default, then optionally replace it if the WAF implements experimental.WAFWithOptions. There was no actual "duplicated creation of transactions" issue in the original code.

I've reverted http/middleware.go to the correct pattern in commit 8a5604e.