[fix] Use cursor.description to detect result-returning queries

[jonasbrami]

Replace brittle sql_upper.startswith() keyword check with cursor.description, which reliably detects any statement that returns a result set (SELECT, SHOW, DESCRIBE, EXPLAIN, CTEs, etc.) without maintaining a hardcoded list of SQL keywords.

[catpineapple]

Blocking: missing regression tests.

This classification branch has already caused Issue #62 Bug 5 once, and the cases motivating this PR (leading comments, parenthesized SELECT) aren't covered by tests either. Without tests, the same bug class will regress again.

Please add regression tests covering at least:

• /* c */ SELECT 1 — leading SQL comment
• WITH t AS (SELECT 1) SELECT * FROM t — CTE, guards Issue [some bugs] #62 Bug 5
• SHOW TABLES / DESC <tbl> / EXPLAIN SELECT 1
• INSERT / UPDATE / CREATE TABLE — locks in the else branch behavior

[catpineapple]

Thank you for your contribution, but some additional test cases are needed to ensure the robustness of doris MCP.