Fix race condition in idp-cli generate-manifest --test-set (#193)

[rstrahan]

Summary

Fixes #193 — Race condition in idp-cli generate-manifest --test-set that causes test sets to be marked as FAILED with "Missing baseline files" errors when the UI is open during CLI uploads.

Root Cause

When the CLI uploads input files then baselines sequentially to S3, the test set resolver's auto-detection (triggered by UI polling via getTestSets) can validate the test set before all baseline files are uploaded. This creates a premature FAILED DynamoDB entry. The CLI's subsequent resolver call then skips re-validation because the entry already exists.

Solution

Implemented a .uploading S3 marker file protocol:

1. CLI places marker — Before uploading files, the CLI creates {test_set}/.uploading in the test set bucket
2. Resolver checks marker — _is_valid_test_set_structure() checks for the marker via head_object and skips folders that have it
3. CLI removes marker — After all uploads complete, the CLI deletes the marker, then invokes the resolver for registration

Changes

File	Change
nested/appsync/src/lambda/test_set_resolver/index.py	Added .uploading marker check in _is_valid_test_set_structure()
lib/idp_cli_pkg/idp_cli/cli.py	Added marker create/delete in generate_manifest() and _create_test_set_from_manifest()
CHANGELOG.md	Added fix entry under [Unreleased]

Backward Compatibility

• Fully backward compatible — Old CLIs without the marker continue to work (resolver only adds an additional skip check)
• No infrastructure changes — No new DynamoDB fields, Lambda APIs, or AppSync schema changes
• Fail-safe — If CLI crashes mid-upload, the marker persists and the test set stays hidden from auto-detection (correct behavior)

[rstrahan]

Can you review / verify this fix, @kaleko ? Many tx.