Rename `CLI_ASYNC_DDL` to `DDL_EXECUTION_MODE` enum variable

[apstndb]

Rename CLI_ASYNC_DDL to DDL_EXECUTION_MODE enum variable

Context

Part of #47 (java-spanner/go-sql-spanner compatibility). Currently CLI_ASYNC_DDL is a boolean variable that doesn't match upstream naming or behavior.

Upstream References

• go-sql-spanner v1.24.0 (connection_properties.go:328-389): ddl_execution_mode enum (SYNC/ASYNC/ASYNC_WAIT) + ddl_async_wait_timeout (default 10s)
• java-spanner: ddlExecutionMode connection-time property

Changes Required

Variable Rename and Type Change

• Rename CLI_ASYNC_DDL (bool) → DDL_EXECUTION_MODE (enum: SYNC, ASYNC, ASYNC_WAIT)
• SYNC = current false (default)
• ASYNC = current true
• ASYNC_WAIT = new mode (wait for DDL completion with timeout)

New Companion Variable

• Add DDL_ASYNC_WAIT_TIMEOUT duration variable (default 10s, per go-sql-spanner)
• Only effective when DDL_EXECUTION_MODE = ASYNC_WAIT

CLI Flag Update

• Update --async CLI flag to map to DDL_EXECUTION_MODE = ASYNC

Backward Compatibility

• No backward compatibility required per project philosophy

Acceptance Criteria

• SET DDL_EXECUTION_MODE = 'SYNC' / 'ASYNC' / 'ASYNC_WAIT' works
• SHOW VARIABLE DDL_EXECUTION_MODE works
• SET DDL_ASYNC_WAIT_TIMEOUT = '30s' works
• SHOW VARIABLE DDL_ASYNC_WAIT_TIMEOUT works
• --async CLI flag sets mode to ASYNC
• ASYNC_WAIT mode waits for DDL completion with configurable timeout
• CLI_ASYNC_DDL is fully removed
• make check passes