fix(cli): gen2-migration refactor creates snapshot filenames that are too long for windows machines (#14844)

fix(cli): gen2-migration refactor creates snapshot filenames that are too long for windows machines

Replace long snapshot filenames with 10-character SHA-256 hashes
and record the hash-to-original-name mapping in a separate
filename-mapping.json file. This avoids Windows MAX_PATH (260
char) errors when snapshot filenames include deeply nested
CloudFormation stack names.

Also rename OUTPUT_DIRECTORY to REFACTOR_SNAPSHOT_OUTPUT_DIRECTORY
for clarity and move the output path under .amplify/ to align
with the project convention.

Updated all migration app snapshots to use the new hashed format.
---
Prompt: implement the "writeRefactorSnapshotFile" such that
hashes the filename to 10 characters and records the mapping
from the hash to the original filename in a separate mapping
file that is updated each time it is invoked.

Author: iliapolo