chore(ci): replace Busy Engineer's Workshop validation with in-repo install-and-import smoke test

The Busy Engineer's Document Bucket workshop
(https://github.com/aws-samples/busy-engineers-document-bucket and its
host https://document-bucket.awssecworkshops.com) is being torn down
(ticket P417981931). The release-validate CodeBuild step depended on
cloning that repo and running its mocked pytest suite to confirm the
freshly-published aws-encryption-sdk wheel still works for a downstream
consumer.

The workshop tests are fully mocked (KMS, S3, DDB, and decrypt are all
mock.Mock/MagicMock), so the only real signal the step produced was:
1. the published wheel installs on a supported Python, and
2. `aws_encryption_sdk` imports, including legacy public symbols like
   `StrictAwsKmsMasterKeyProvider`.

This change reproduces exactly that signal without the workshop
dependency: pip install the released $VERSION, then import the package
and the legacy symbol. Retry behavior is preserved.

Author: lucasmcdonald3

codebuild/release/validate.yml

@@ -2,25 +2,20 @@ version: 0.2
 
 phases:
   install:
-    commands:
-      - pip install "tox < 4.0"
     runtime-versions:
       python: latest
-  pre_build:
     commands:
-      - git clone https://github.com/aws-samples/busy-engineers-document-bucket.git
-      - cd busy-engineers-document-bucket/exercises/python/encryption-context-complete
-      - sed -i "s/aws_encryption_sdk/aws_encryption_sdk==$VERSION/" requirements-dev.txt
-      - pyenv install --skip-existing 3.8.12
-      - pyenv local 3.8.12
-      - pip install "tox < 4.0"
+      - python -m pip install --upgrade pip
   build:
     commands:
       - NUM_RETRIES=3
       - |
         while [ $NUM_RETRIES -gt 0 ]
         do
-          tox -re test
+          pip install "aws-encryption-sdk==$VERSION" \
+            && python -c "import aws_encryption_sdk; \
+                          from aws_encryption_sdk import StrictAwsKmsMasterKeyProvider; \
+                          print('aws-encryption-sdk', aws_encryption_sdk.__version__, 'installed and imports cleanly')"
           if [ $? -eq 0 ]; then
             break
           fi