Restore default shard_path_type to FIXED for snapshot repositories

[mango766]

Description

Restores the default value of shard_path_type in BlobStoreRepository from HASHED_PREFIX back to FIXED.

PR #18163 changed this default, which causes snapshot shard blobs to be uploaded outside the configured base_path in storage services like S3/MinIO. This is a breaking change for anyone upgrading from 2.x to 3.x — incremental backups fail because the new path layout is incompatible with existing repositories. The change also went in without a changelog entry.

The fix aligns the repository setting default with the existing IndexId.DEFAULT_SHARD_PATH_TYPE constant, which is already FIXED. Users who want HASHED_PREFIX can still set it explicitly when registering their repository.

Related Issues

Resolves #20643

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[github-actions]

PR Reviewer Guide 🔍

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🧪 No relevant tests

🔒 No security concerns identified

✅ No TODO sections

🔀 No multiple PR themes

⚡ Recommended focus areas for review Breaking Change Changing the default shard_path_type from HASHED_PREFIX back to FIXED is a breaking change for users who upgraded to 3.x and already created repositories with the HASHED_PREFIX default. Those users will now have a mismatch between existing snapshot data (stored under hashed paths) and the new default, potentially causing incremental backup failures. Consider whether a migration path or warning is needed for users already on 3.x with HASHED_PREFIX repositories. public static final Setting<PathType> SHARD_PATH_TYPE = new Setting<>( "shard_path_type", PathType.FIXED.toString(), PathType::parseString );

[github-actions]

✅ Gradle check result for ea78a17: SUCCESS

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.24%. Comparing base (06539a4) to head (ea78a17).

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #20940      +/-   ##
============================================
- Coverage     73.27%   73.24%   -0.03%     
+ Complexity    72497    72442      -55     
============================================
  Files          5819     5819              
  Lines        331352   331352              
  Branches      47875    47875              
============================================
- Hits         242784   242699      -85     
- Misses        69065    69151      +86     
+ Partials      19503    19502       -1     

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