fix: Upload SDK files via input_files for example validation

Examples import SDK libraries (from un import execute_code, etc.) which
aren't available in the sandbox. Instead of making examples standalone
(which defeats the purpose), upload SDK source files via the API's
input_files parameter and rewrite import paths to /tmp/input/.

Changes:
- validate-examples.sh: detect SDK src dir, base64-encode files into
  input_files JSON, rewrite Python/JS/Ruby/PHP import paths, pipe
  request body via stdin to avoid arg length limits
- validate-examples.sh: add JUnit XML generation (science-results.xml)
- .gitlab-ci.yml: remove allow_failure from science-validate-examples
  and validate-examples jobs
- .gitignore: add science-results/ (CI artifacts, not source)
- git rm science-results/ (committed "100% pass" was a lie)

.gitlab-ci.yml

@@ -168,7 +168,6 @@ science-validate-examples:
     paths:
       - science-results/
     expire_in: 30 days
-  allow_failure: true
   only:
     - main
     - /^\d+\.\d+\.\d+$/
@@ -231,7 +230,6 @@ validate-examples:
     paths:
       - science-results/
     expire_in: 30 days
-  allow_failure: true
   only:
     - main
     - /^\d+\.\d+\.\d+$/