[spark] Support FROM (query) export in COPY INTO location

[JunRuiLee]

Purpose

Extend COPY INTO <location> (export) to accept an inline query as the source, not just a table:

COPY INTO '/export/active_users/'
FROM (SELECT id, name FROM my_db.users WHERE active = TRUE)
FILE_FORMAT = (TYPE = CSV, HEADER = TRUE);

Previously only FROM table_name was supported. The inline query is parsed through the
session (Paimon) parser, so it behaves exactly like the same query run via spark.sql,
including Paimon parser rules such as the v1 function rewrite.

Design notes:

• Grammar: a parenBlock rule captures the parenthesized source with balanced
  parentheses, so nested parens (e.g. WHERE id IN (1, 2)) are matched correctly. The raw
  text is re-parsed by the AST builder.
• Read-only enforcement: any read-only query is allowed (SELECT, WITH ... SELECT,
  VALUES, ...); statements with side effects are rejected by inspecting the parsed plan for
  Command / ParsedStatement / InsertIntoDir nodes. DDL/DML such as DROP, INSERT, and
  INSERT OVERWRITE DIRECTORY are rejected before any execution, so they cannot run.
• Source modeling: the source (table name vs. query) is modeled as a sealed ADT in both
  the logical command and the physical exec, so impossible states cannot be constructed.
• Row count: rows_written is counted before the write; for a non-deterministic query it
  may differ from the actual output. This is documented; the result is intentionally not
  staged, so the export does not consume extra executor disk.

This is part of #8005.

Tests

• CopyIntoTestBase: CSV/JSON/Parquet export from a query, aggregation, nested parentheses,
  OVERWRITE = TRUE, VALUES source, empty-query rejection, and rejection of
  side-effecting statements (with assertions that the source table is untouched and no files
  are written).
• PaimonV1FunctionTestBase: exporting FROM (SELECT <v1_function>(...)) resolves correctly
  through the session parser.

[JingsongLi]

cc @Zouxxyy to take a look.