aa: improve code quality by propagating gRPC server errors

[1hcoj]

When the configured gRPC port is already in use, start_grpc_service() returns an error, but main discards it in tokio::select!. As a result, grpc-aa logs AA exits. and exits with status 0 despite failing to start the server.

This small code quality improvement propagates the server error with the socket address as context, so startup failures report their cause and return a nonzero exit status. It also changes the pre-bind log from listening to Starting to avoid claiming readiness before binding.

Adds a server unit test for an occupied port and executable integration tests for an occupied port and an invalid socket address. The tests require no TEE hardware or KBS service.

Validation:

cargo test --locked -p attestation-agent --no-default-features \
  --features bin,grpc --bin grpc-aa --test grpc_startup -- --nocapture
cargo clippy --locked -p attestation-agent --no-default-features \
  --features bin,grpc --bin grpc-aa --test grpc_startup -- -D warnings
cargo fmt --all -- --check