keyspace: Remove the "apiv2-no-prefix" feature

[pingyu]

Summary

• Remove the apiv2-no-prefix Cargo feature and make API V2 “no prefix” behavior always available.

What changed

• Delete apiv2-no-prefix feature from Cargo.toml and remove it from Makefile feature/test flags (Cargo.toml, Makefile).
• Make Keyspace::ApiV2NoPrefix and TransactionClient::new_with_config_api_v2_no_prefix unconditional (no longer #[cfg(feature = "apiv2-no-prefix")]) (src/request/keyspace.rs, src/transaction/client.rs).
• Mark Keyspace as #[non_exhaustive].
• Update README dependency version to 0.4 (README.md:20).

Notes

• Breaking changes are expected for the 0.4.0 bump

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: coocood, iosmanthus

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-01-20 14:12:56.900573318 +0000 UTC m=+503204.514530164: ☑️ agreed by iosmanthus.
• 2026-01-20 15:39:57.131346019 +0000 UTC m=+508424.745302865: ☑️ agreed by coocood.