fix: return Doris column comments in table schema

[FreeOnePlus]

Summary

• use SHOW FULL COLUMNS for the MCP table-schema path
• preserve existing safe two-part and catalog-qualified identifier handling
• map Doris column comments into the existing comment response field
• add unit coverage for internal and external catalogs
• add a real Doris regression through HTTP and subprocess stdio transports

Root cause

The active get_table_schema MCP path queried Doris with DESCRIBE but then
attempted to read a Comment field. Doris DESCRIBE does not return that
field, so column comments were always empty even when they existed.

SHOW FULL COLUMNS returns the same schema fields used by the current response
mapping plus the actual Comment value.

Validation

• uv run pytest -q: 1088 passed, 68 skipped; coverage 57.95%
• focused metadata tests: 41 passed
• real Doris HTTP and subprocess stdio suite: 9 passed, 2 optional monitoring tests skipped
• real Doris regression creates a commented column and verifies the MCP result
• uv run ruff check .
• uv run mypy doris_mcp_server
• CI-equivalent Bandit scan

Fixes #81