[BUG] ModifyResponsePlugin always parses and re-serializes response body as JSON even for header-only configurations

- severity: Medium
- files: `shenyu-plugin/shenyu-plugin-modify-response/.../ModifyResponsePlugin.java:148-171`
- description: In `writeWith`, `modifyBody(bytes)` is unconditionally called regardless of whether any body modification rules are configured. `JsonPath.parse(jsonValue)` is always called and `context.jsonString()` always re-serializes. For non-JSON responses (XML, HTML, binary), `JsonPath.parse` throws → `ShenyuException`.
- impact: (a) Non-JSON responses crash the plugin even when only header modifications are configured. (b) Re-serialization can change whitespace/key ordering for valid JSON.
- suggested_fix: Short-circuit `modifyBody` when all body-key lists are empty/null; add a content-type guard.
- confidence: Medium
- 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)._

Start in shenyu-plugin/shenyu-plugin-modify-response/.../ModifyResponsePlugin.java:148-171, read writeWith and modifyBody, and trace the body-key lists and content type handling. Verify that header-only configurations pass through non-JSON bodies without parsing, while configured JSON responses retain the intended modifications without unnecessary re-serialization.

Written by the indexing model from the issue text.