[Task] Add read_only connection mode support for standalone clients

[prateek-kumar-improving]

Description

Expose the read_only connection option for standalone clients, matching the implementation already available in Java, Python, Node, Go, and C#.

read_only is a standalone connection mode (distinct from the read_from routing strategy). When enabled, the client:

• Skips primary-node detection during connection initialization, and
• Rejects write commands.

This is useful for connecting to replica-only deployments. It maps to protobuf field read_only = 26 on ConnectionRequest.

Note: read_only is mutually exclusive with NodeDiscoveryMode.DISCOVER_ALL (glide-core rejects the combination because DISCOVER_ALL requires the INFO REPLICATION call that read-only mode skips). Implementing read_only would also unblock porting the read_only + DISCOVER_ALL rejection test that was intentionally omitted in #322 (NodeDiscoveryMode) because PHP had no way to set the flag.

Checklist

1. Add a read_only parameter to ValkeyGlide::connect() (optional, defaults to false).
2. Add a read_only field to the standalone client configuration and constructor-params structs.
3. Wire the config to protobuf field read_only = 26 on ConnectionRequest (standalone only).
4. Validate incompatible combinations consistent with glide-core (e.g. read_only + AZ-affinity read_from strategies; read_only + DISCOVER_ALL).
5. Add serialization tests (ConnectionRequestTest) asserting the protobuf field is set.
6. Add integration tests, including the read_only + DISCOVER_ALL rejection case and replica-only read behavior.
7. Update documentation (README + stub docblocks) and CHANGELOG.

Additional Notes

• Reference implementations:
  • Go: ClientConfiguration.readOnly -> request.ReadOnly in go/config/config.go.
  • C#: NodeDiscoveryMode docs note it is "mutually exclusive with read-only mode".
  • Rust core validation lives in glide-core/src/client/standalone_client.rs (read-only mode is not compatible with ...).
• This is a standalone-only feature; cluster clients do not use read_only.
• Related: [Task] Add NodeDiscoveryMode support for standalone clients #302 / feat(standalone): add NodeDiscoveryMode support for standalone clients #322 (NodeDiscoveryMode) documented this gap as a limitation.