[BUG] RequestConfigHelper.getUrlTemplate/getMethod NPE when requestTemplate/url absent

- severity: Medium
- files: `shenyu-plugin/shenyu-plugin-mcp-server/src/main/java/org/apache/shenyu/plugin/mcp/server/request/RequestConfigHelper.java:81-83,90-93`
- description: `getUrlTemplate()` is `getRequestTemplate().get("url").getAsString()` with no null checks. `getRequestTemplate()` returns null when config lacks `requestTemplate`; `.get("url")` returns null when `url` is absent. Either case NPEs on every tool invocation.
- impact: Tool calls with missing `requestTemplate`/`url` fail with opaque NPE.
- suggested_fix: Null-check and throw descriptive `IllegalArgumentException`.
- confidence: High
- related_existing: none

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_Identified during the 2026-08-02 deep re-scan; full list in [`docs/scan2-2026-08-02/06-medium-tiers.md`](docs/scan2-2026-08-02/06-medium-tiers.md)._