Nuclei: Environment Variable Disclosure via Response-Derived Data in DAST/Fuzz Mode

A vulnerability in Nuclei's DAST/fuzz expression evaluation path allows a malicious target server to trigger disclosure of scanner-host environment variables when the `-env-vars` / `-ev` option is explicitly enabled. This is an incomplete fix for [CVE-2026-41645](https://github.com/projectdiscovery/nuclei/security/advisories/GHSA-jm34-66cf-qpvr) / GHSA-jm34-66cf-qpvr. The original fix hardened `expressions.Evaluate()` to be single-pass within one call, but did not address callers that invoked evaluation multiple times on substituted output in the DAST/fuzz pipeline. **Affected Component** The issue is in the DAST/fuzz payload evaluation path (`pkg/fuzz/parts.go`) and the shared template rendering boundary. When a multi-step template captures response data via an internal extractor and reuses it in a subsequent fuzz step, the fuzz evaluator could treat the substituted response content as fresh template syntax on a second evaluation pass. **Description** In DAST/fuzz mode, payload evaluation previously ran expression substitution more than once on the same value. Response-derived content captured by an `internal: true` extractor in a prior protocol step could flow into a fuzz payload and be reinterpreted as DSL/helper syntax on a subsequent pass. When `-env-vars` (`-ev`) is enabled, environment variables are merged into the template variable map. A malicious target can return response data containing expressions like `{{env_var_name}}` which, when reused in a subsequent fuzz st