fix: make default query row limit configurable

[FreeOnePlus]

Summary

• add DEFAULT_RESULT_ROWS as the per-query default for exec_query
• keep MAX_RESULT_ROWS as the independent deployment ceiling
• expose the configured default consistently in the MCP tool schema and runtime handler
• document the setting for environment and Docker Compose deployments

Root cause

MAX_RESULT_ROWS already controlled the maximum allowed request, but both the
tool schema and handler hardcoded an omitted max_rows argument to 100.
Increasing the deployment ceiling therefore did not change the default result
size.

User impact

Operators can now set, for example:

MAX_RESULT_ROWS=10000
DEFAULT_RESULT_ROWS=1000

Clients that omit exec_query.max_rows receive up to 1000 rows, while explicit
requests remain bounded by the 10000-row deployment ceiling.

Validation

• uv run pytest -q: 1088 passed, 68 skipped; coverage 57.94%
• real Doris HTTP and subprocess stdio suite: 9 passed, 2 optional monitoring tests skipped
• real Doris regression verifies omitted max_rows uses DEFAULT_RESULT_ROWS
• uv run ruff check .
• uv run mypy doris_mcp_server
• uv run bandit -r doris_mcp_server -c pyproject.toml
• uv build
• docker compose config --quiet

Fixes #66
Fixes #72