Fix SnowflakeHook transaction support: multi-statement SQL and AUTOCOMMIT

[kaxil]

Fixes two longstanding issues with SnowflakeHook.run() that broke Snowflake transaction support:

1. Multi-statement SQL with split_statements=False (SnowflakeHook.run() doesn't accept multiple statements without split_statements #48233): When sending a multi-statement block like BEGIN; INSERT ...; COMMIT; as a single cursor.execute() call, Snowflake rejects it because the connector defaults MULTI_STATEMENT_COUNT=1. Now passes num_statements=0 (auto-detect) to cursor.execute() when split_statements=False.

2. AUTOCOMMIT session parameter override (Session_parameter AUTOCOMMIT is not set in Snowflake connection #30236): Users who set session_parameters={"AUTOCOMMIT": True} on their connection had it immediately overridden by set_autocommit(conn, False) in run(). Now respects the session parameter when autocommit=False (the default).

Design rationale

Why override _run_command() instead of modifying run() inline? The base DbApiHook._run_command() handles logging, lineage tracking, and row counts. Overriding it in SnowflakeHook keeps the num_statements logic close to cursor.execute() while preserving all side effects. The override drops the psycopg list-to-tuple conversion since Snowflake never uses psycopg.

Why case-insensitive AUTOCOMMIT check? Snowflake session parameters are case-insensitive, so users may write {"autocommit": True} or {"AUTOCOMMIT": True}. The check normalizes to uppercase.

Why set conn.autocommit_mode = True in the else branch? When we skip set_autocommit() to respect the session parameter, get_autocommit() would return False (since autocommit_mode was never set), causing a redundant conn.commit() at the end of run(). Setting the mode explicitly avoids this.

Why _get_static_conn_params instead of _get_conn_params()? The static version is a @cached_property and doesn't trigger OAuth token refresh, avoiding surprise HTTP requests from what should be a simple config check.

Validated against real Snowflake

All three scenarios pass against a real Snowflake instance:

• Multi-statement block (CREATE + INSERT + SELECT + DROP) with split_statements=False
• Transaction block (CREATE + BEGIN + INSERT x3 + COMMIT) with split_statements=False
• Backward compat: split_statements=True (default) still works with separate query IDs

Closes #48233
Closes #30236

[kaxil]

cc @josh-fell