fix(r): schema-qualify CTE table refs to avoid circular references

[cpsievert]

Problem

Four TblSqlSource tests fail with a "circular reference to CTE" error on DuckDB 1.3.0+ (and SQLite):

Error: Binder Error: Circular reference to CTE "test_table", There are two possible solutions. 
1. use WITH RECURSIVE to use recursive CTEs. 
2. If you want to use the TABLE name "test_table" the same as the CTE name, please explicitly add "SCHEMA" before table name. You can try "main.test_table"

This happens when a TblSqlSource is created from a transformed tbl_sql (e.g., dplyr::filter(), dplyr::select()). The class wraps the transformed tbl's SQL as a CTE named after the table:

WITH "test_table" AS (
  SELECT test_table.* FROM test_table WHERE ...
)
SELECT * FROM test_table

DuckDB 1.3.0+ and SQLite reject this because the CTE name ("test_table") shadows the physical table (test_table) referenced in the CTE body, creating a circular reference.

Fix

Schema-qualify the table references in the CTE body so they resolve to the physical table, not the CTE:

WITH "test_table" AS (
  SELECT test_table.* FROM main.test_table WHERE ...
)
SELECT * FROM test_table

Two helper functions are added to TblSqlSource.R:

• get_current_schema(conn) — tries SELECT current_schema() (DuckDB, PostgreSQL) and falls back to "main" for databases that don't support it (SQLite).
• qualify_cte_table_refs(conn, cte_body) — uses a regex to prefix FROM/JOIN table references with the schema name. Skips already-qualified names and subquery FROM clauses.

Verification

All 64 test-TblSqlSource.R tests pass (previously 4 failed). Full devtools::test() shows no new failures or regressions.

[cpsievert]

Closing this PR — the 4 TblSqlSource test failures it addresses are local-only, caused by an outdated local DuckDB (1.3.0).

CI uses DuckDB 1.5.5, which already fixed this circular CTE reference bug upstream in DuckDB 1.5.0 (PR #19116), resolving issue #6389. The schema-qualification workaround in this PR adds complexity to work around a DuckDB bug that no longer exists in the versions CI and most users run.

If support for older DuckDB (< 1.5.0) becomes a requirement, this approach could be revisited. Thanks to @cpsievert for the investigation.