feat(cloud-storage): add pluggable cloud storage support for distributed HugeGraph persistence

[vaijosh]

Why

This PR introduces a unified cloud storage capability for HugeGraph’s distributed stack, so store data can be synchronized to cloud object storage for stronger durability and recovery options.

The focus is not a provider-specific implementation detail, but a cloud-capable architecture with a default S3-compatible provider and extension points for additional providers.

What this PR delivers

1) Cloud storage support in distributed runtime

• Enables cloud sync for store-side RocksDB in distributed deployments (backend=hstore flow).
• Supports both:
  • cloud-first mode: commit waits for cloud sync (stronger durability)
  • async mode: local/Raft commit first, periodic cloud reconciliation

2) Pluggable provider architecture

• Introduces provider/client abstractions for cloud backends.
• Uses ServiceLoader discovery so new cloud providers can be added via plugin JARs.
• Keeps built-in s3 provider as default (S3-compatible API model), while allowing future providers without core rewrites.

3) Cloud-neutral configuration model

• Standardizes configuration naming around cloud-neutral keys (cloud_*).
• Aligns server-side propagation and store-side consumption of cloud settings.
• Cleans up older S3-specific naming to keep the config surface consistent and provider-agnostic.

4) Operational docs and examples

• Updates architecture and usage docs to describe cloud storage behavior consistently.
• Adds/updates sample plugin guidance and SPI wiring for provider developers.
• Aligns docker/dev scripts and templates with cloud storage configuration.

User Impact

• Resilience against Ephemeral Infrastructure: By treating cloud object storage as the decoupled, durable source of truth, HugeGraph can seamlessly adapt to cloud-native environments (e.g., Kubernetes). If an instance or pod is unexpectedly terminated, rescheduled, or suffers from local disk/EBS detachment, the system prevents catastrophic data loss. New instances can instantly rehydrate or recover their state directly from the cloud checkpoint.
• Flexible Durability SLAs: Operators running distributed deployments (backend=hstore) can choose their optimal trade-off between throughput and durability:
  • Cloud-First Mode (sync): Guarantees zero data loss by ensuring local Raft commits inline with cloud storage flushes before returning success to the caller.
  • Asynchronous Mode (async): Minimizes latency by performing background reconciliation to the cloud storage bucket within a bounded time horizon.

Validation

• Maven compile/build verification on affected modules.
• Manual Validation and smoke test validation check RocksDB-cloud.md for more details.

Reviewer focus areas

• Cloud config key consistency across server/store/docs
• Provider abstraction and ServiceLoader integration
• Cloud-first vs async durability semantics
• Recovery/rehydration behavior and operational clarity

[codecov]

Codecov Report

❌ Patch coverage is 0% with 662 lines in your changes missing coverage. Please review.
✅ Project coverage is 29.84%. Comparing base (39dfb2d) to head (4aed5da).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
...java/org/apache/hugegraph/pd/PartitionService.java	0.00%	176 Missing ⚠️
...gegraph/store/partition/PartitionLeaseManager.java	0.00%	104 Missing ⚠️
...hugegraph/store/partition/LeaseEpochValidator.java	0.00%	80 Missing ⚠️
...egraph/store/raft/PartitionLeaseStateListener.java	0.00%	52 Missing ⚠️
...va/org/apache/hugegraph/store/PartitionEngine.java	0.00%	46 Missing ⚠️
...g/apache/hugegraph/store/pd/DefaultPdProvider.java	0.00%	35 Missing ⚠️
...pache/hugegraph/pd/meta/PartitionBucketRecord.java	0.00%	33 Missing ⚠️
.../java/org/apache/hugegraph/pd/client/PDClient.java	0.00%	31 Missing ⚠️
...va/org/apache/hugegraph/pd/meta/StoreInfoMeta.java	0.00%	26 Missing ⚠️
...rg/apache/hugegraph/pd/meta/MetadataKeyHelper.java	0.00%	22 Missing ⚠️
... and 8 more

❗ There is a different number of reports uploaded between BASE (39dfb2d) and HEAD (4aed5da). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (39dfb2d)	HEAD (4aed5da)
	2	1

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3061      +/-   ##
============================================
- Coverage     34.89%   29.84%   -5.06%     
- Complexity      338      375      +37     
============================================
  Files           803      808       +5     
  Lines         68241    69051     +810     
  Branches       8965     9057      +92     
============================================
- Hits          23815    20606    -3209     
- Misses        41826    46069    +4243     
+ Partials       2600     2376     -224     

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