[BUG] SseEventFormatter uses default-charset getBytes() for SSE

- severity: Medium
- files: `shenyu-plugin/shenyu-plugin-mcp-server/src/main/java/org/apache/shenyu/plugin/mcp/server/transport/SseEventFormatter.java:76,89`
- description: `formatEvent`/`formatComment` build the SSE frame then `exchange.getResponse().bufferFactory().wrap(formattedEvent.getBytes())` — platform default charset. SSE is a UTF-8 text stream; MCP JSON payloads frequently contain non-ASCII.
- impact: Non-ASCII SSE event data is corrupted on non-UTF-8 default charsets.
- suggested_fix: Use `formattedEvent.getBytes(StandardCharsets.UTF_8)`.
- 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)._