[DRIVER-789] Validate SELECT without FROM support

Jira: https://scylladb.atlassian.net/browse/DRIVER-789
Parent epic: https://scylladb.atlassian.net/browse/DRIVER-782
Server-side issue: https://github.com/scylladb/scylladb/issues/24070
Server-side PR: https://github.com/scylladb/scylladb/pull/29028

This issue tracks the cpp-rs-driver: Validate SELECT without FROM support task from DRIVER-782.

Scope:

• Verify that SELECT 1 executes successfully without keyspace or table metadata.
• Verify that SELECT now() executes successfully without keyspace or table metadata.
• Verify result metadata and decoded values for literal and function selectors where the driver exposes them.
• Cover prepared and non-prepared execution paths where relevant for this driver.
• Skip cleanly or feature-detect when the server does not support SELECT without FROM.

Acceptance criteria:

• Integration tests cover SELECT 1 and SELECT now().
• Returned values are decoded where exposed by the driver.
• Tests do not require a keyspace or table.
• Driver-specific limitations or unsupported execution paths are documented.

Start by locating the driver's existing integration tests for literal and function selectors, then inspect the prepared and non-prepared execution paths. Add coverage for SELECT 1 and SELECT now() without a keyspace or table, including decoded values and result metadata where exposed. Confirm unsupported server capabilities are skipped or feature-detected, and document driver-specific limitations.