Align READONLY and READWRITE with Redis cluster semantics

[Vasileios Zois (vazois)]

Description

Aligns Garnet's cluster READONLY and READWRITE behavior with Redis semantics:

• Replica reads redirect to the primary by default; READONLY enables replica-local reads for slots owned by its primary, and READWRITE restores default redirection.
• Client writes remain redirected from replicas regardless of connection read mode.
• Internal AOF replay write access is isolated from client session state, including replica FLUSHDB and FLUSHALL handling.
• CONFIG GET slave-read-only reports the fixed replica write policy as yes.
• Cluster test connections issue READONLY after connect and reconnect so existing replica-local validation remains explicit and stable.

Fixes #2086
Fixes #2087

[Copilot]

🟡 Changes recommended

The public IClusterSession source-breaking change must be preserved compatibly or explicitly documented as breaking.

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Pull request overview

Aligns replica read/write behavior with Redis semantics while isolating internal AOF replay permissions.

Changes:

• Makes READONLY control replica-local reads only; client writes always redirect.
• Adds an internal replay-write capability and fixed slave-read-only=yes.
• Updates tests and documentation.

Metadata: Description is accurate. Recommended title: [Cluster] Align READONLY and READWRITE with Redis cluster semantics.

File summaries

File	Description
website/docs/commands/cluster.md	Clarifies command semantics.
website/docs/cluster/replication.md	Updates replica usage guidance.
test/standalone/Garnet.test/RespConfigTests.cs	Verifies fixed config value.
test/standalone/Garnet.test/RespAdminCommandsTests.cs	Updates config expectation.
test/cluster/Garnet.test.cluster/ClusterTestUtils.cs	Enables replica reads in tests.
test/cluster/Garnet.test.cluster/ClusterRedirectTests.cs	Tests read-mode redirects.
test/cluster/Garnet.test.cluster.replication/ReplicationTests/ClusterReplicationBaseTests.cs	Tests failover write rejection.
libs/server/ServerConfig.cs	Returns fixed replica policy.
libs/server/Resp/BasicCommands.cs	Restricts flushes to internal replay.
libs/server/Config/RuntimeServerConfig.cs	Updates config documentation.
libs/server/Cluster/IClusterSession.cs	Revises session capability API.
libs/server/AOF/AofProcessor.cs	Grants internal replay writes.
libs/resources/RespCommandsDocs.json	Corrects READWRITE summary.
libs/cluster/Session/SlotVerification/ClusterSlotVerify.cs	Separates read and write permissions.
libs/cluster/Session/MigrateCommand.cs	Updates locality argument naming.
libs/cluster/Session/ClusterSession.cs	Stores separate session flags.
libs/cluster/Server/Replication/ReplicaOps/ReplicaDisklessSync.cs	Enables diskless replay writes.
libs/cluster/Server/Replication/ReplicaOps/ReplicaDiskbasedSync.cs	Enables disk-based replay writes.
libs/cluster/Server/ClusterManagerSlotState.cs	Updates locality calls.
libs/cluster/Server/ClusterConfig.cs	Renames replica-read locality control.

Review details

• Files reviewed: 20/20 changed files
• Comments generated: 1
• Review effort level: Balanced

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