Enables integration tests

[johnnyt]

Integration tests now run by default:

• Removed :integration from default ExUnit exclusions
• KMS integration tests skip gracefully without AWS credentials
• Developers can still exclude locally: mix test --exclude integration
• CI with AWS credentials runs full integration suite

Added comprehensive test coverage:

• 271 lines of streaming error tests (authentication failures, commitment mismatches, signature verification, trailing bytes)
• 161 lines of CMM dispatch tests for RequiredEncryptionContext and Caching CMMs wrapping patterns
• Removed 42 coveralls-ignore markers from production code

Coverage threshold adjusted to 92%:

• Updated codecov.yml and coveralls.json
• Current coverage: 92.6% (exceeds threshold)
• Remaining gaps require complex error injection scenarios

Closes #68

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff             @@
##             main      #70      +/-   ##
==========================================
- Coverage   94.15%   92.68%   -1.48%     
==========================================
  Files          40       40              
  Lines        1369     1435      +66     
==========================================
+ Hits         1289     1330      +41     
- Misses         80      105      +25     

Files with missing lines	Coverage Δ
..._encryption_sdk/cmm/required_encryption_context.ex	87.80% <100.00%> (+0.62%)	⬆️
...ib/aws_encryption_sdk/keyring/kms_client/ex_aws.ex	78.37% <ø> (-21.63%)	⬇️
lib/aws_encryption_sdk/stream.ex	72.88% <ø> (-17.82%)	⬇️
lib/aws_encryption_sdk/stream/decryptor.ex	88.54% <ø> (-4.23%)	⬇️

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