feat: supports time_zone and show,tql eval etc.

[killme2008]

• Supports time_zone setting, but depends on SET @@session.time_zone='-7:00' doesn't work for MySQL greptimedb#6207
• call_tool supports show, desc table, tql, explain and show databases statements.

[Copilot]

Pull Request Overview

This PR adds support for a new configuration option (time_zone) and extends SQL tool functionalities to include queries like SHOW DATABASES, TQL, DESC TABLE, and EXPLAIN.

• Adds time_zone handling to configuration and server connections.
• Introduces new tests for SHOW DATABASES queries and validates time_zone settings from different sources.
• Updates the DatabaseServer's call_tool method to handle additional SQL commands and adjusts configuration parsing.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/test_server.py	Adds time_zone support and a new test for SHOW DATABASES queries.
tests/test_config.py	Adds tests asserting time_zone values from defaults, env vars, and CLI args.
src/greptimedb_mcp_server/server.py	Updates the server to include time_zone in connections and enhances query handling.
src/greptimedb_mcp_server/config.py	Introduces the time_zone property and CLI argument.
conftest.py	Adds mocked responses for SHOW DATABASES but contains a duplicate condition issue.