Add subcursor() for interleaved streaming + DML

[rustyconover]

Summary

• Adds subcursor() method that creates a cursor sharing the same Connection and transaction as the parent
• Enables interleaved streaming SELECT + UPDATE/INSERT/DELETE within a single explicit transaction
• Subcursors cannot manage transactions (begin/commit/rollback throw InvalidInputException)
• Closing a subcursor cleans up any suspended query state without affecting the parent

Dependencies

This PR requires the companion core DuckDB PR: duckdb/duckdb#21569 — which adds the enable_suspended_queries setting and the suspend/resume mechanics in ClientContext. Without that PR, subcursor() will create a shared connection but suspension won't activate.

Usage

conn = duckdb.connect('my.db')
conn.execute("SET enable_suspended_queries = true")
conn.execute("BEGIN TRANSACTION")

scanner = conn.subcursor()
updater = conn.subcursor()

result = scanner.execute("SELECT rowid, col FROM t WHERE NOT processed")

while True:
    batch = result.fetchmany(1000)
    if not batch:
        break
    ids = ','.join(str(r[0]) for r in batch)
    updater.execute(f"UPDATE t SET processed=1 WHERE rowid IN ({ids})")

conn.execute("COMMIT")

Changes

• ConnectionGuard: unique_ptr<Connection> → shared_ptr<Connection> with ShareConnection()/GetSharedConnection() accessors
• DuckDBPyConnection::Subcursor(): creates a cursor sharing the parent's Connection
• is_subcursor flag guards Begin()/Commit()/Rollback()
• pybind11 registration for subcursor method

Test plan

• Basic interleaved scan + update pattern
• Multi-table scan pattern (two tables, one updater)
• Transaction guard (subcursor can't begin/commit/rollback)
• Shared transaction visibility (subcursor sees uncommitted data)
• Close doesn't destroy parent connection
• Full stream consumption with periodic interleaved updates
• Explicit transaction required for suspension to work
• Close cancels suspended stream and frees resources
• Close one of multiple suspended streams (others unaffected)
• Abandoned subcursor (GC cleanup via destructor)
• Existing cursor tests unaffected (13/13 pass, 1 pre-existing pytz skip)

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