fix(admin): limit swagger import response body size

[SouthwestAsiaFloat]

#6401

Description

This PR limits the Swagger/OpenAPI import response body size in shenyu-admin.

It adds a configurable shenyu.swagger.max-body-size setting with a default value of 10 MB, rejects oversized Swagger responses both by Content-Length and during streaming, and returns HTTP 400 for
invalid or oversized import requests.

Tests

Added tests in:

• SwaggerImportControllerTest
• SwaggerImportServiceImplTest

The tests cover oversized Swagger response rejection, HTTP 400 responses for invalid import requests, successful small-body imports, unknown Content-Length streaming limits, exact-limit bodies, empty
response bodies, and response charset handling.

Make sure that:

• You have read the contribution guidelines.
• You submit test cases (unit or integration tests) that back your changes.
• Your local test passed ./mvnw clean install -Dmaven.javadoc.skip=true.