fix: enforce request body size limit during buffering in AiProxyPlugin

[wy471x]

Use DataBufferUtils.join with maxSize to prevent OOM from large request bodies. Previously the full body was buffered into memory before the size check ran, allowing a multi-GB payload to exhaust heap. Now DataBufferLimitException is raised during buffering and returned as 413.

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.

Summary

• Replaced DataBufferUtils.join(exchange.getRequest().getBody()) (no max size) with DataBufferUtils.join(exchange.getRequest().getBody(), MAX_REQUEST_BODY_SIZE_BYTES). This makes
  Spring's DataBufferUtils enforce the 5 MB limit during buffering — a DataBufferLimitException is thrown immediately when the limit is exceeded, rather than buffering the entire
  multi-GB body into heap first. Added .onErrorResume(DataBufferLimitException.class, ...) to catch that exception and return HTTP 413. Removed the now-redundant post-buffer size
  check.
• Added testRequestBodyExceedsMaxSize which mocks DataBufferUtils.join to return Mono.error(new DataBufferLimitException(...)) and asserts the response status is 413
  PAYLOAD_TOO_LARGE.

close #6837