[wrossmorrow-pi-bot]

What

Adds the Java SDK's integration tests for full ext_proc exchanges, driven through the
TestingRequestProcessor via the gRPC ExternalProcessor service (the Java analog of the
Go SDK's test/tester.go harness).

Changes

• TestingRequestProcessorIntegrationTest drives a full exchange over a single bidirectional
  gRPC stream through the four envoy phases (request headers → request body → response headers →
  response body) using LoadGen helpers, and asserts on the collected responses.
• LoadGen gains a runExchange(stub, requestBodyJson) overload so the tester can drive exchanges
  with arbitrary action payloads (phase 2 now uses ByteString.copyFromUtf8 for the body).

Tests

• noop exchange completes without error
• request header actions are applied
• response header actions are applied
• request body replacement is applied
• cancel request is applied (immediate response, status 409, body)

./gradlew clean build   # includes the new integration tests

Notes

• Cancel is exercised at the request-body phase and the exchange stops there (the cancel ends the
  stream); phase responses for request-body actions carry the request body mutations
  (ResponseCase.REQUEST_BODY).
• Full compose-stack verification (docker compose up --build) still pending on the maintainer's
  machine (Docker unavailable in the dev sandbox).