fix: reject invalid SQL row limits

[DevChiniwala]

DBConfig accepted row_limit=0 and negative values, while BaseSQLDialect.execute_readonly accepted them too. Invalid limits undermine the bounded-read contract and produce inconsistent slicing behavior. This validates row_limit >= 1 in configuration and rechecks the low-level execution boundary for direct callers. Added config and SQLite dialect regressions for zero and negative values. Validation: 27 config plus SQL safeguard tests passed; no schema or API response changes; targets main.

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[haolpku]

The implementation itself passed the combined local suite, but this PR now conflicts with merged PR #18 in datamind/tests/test_config.py. The resolution is mechanical: import DBConfig, DataConfig, and Settings, and retain both the profile-path tests and test_db_config_rejects_nonpositive_row_limit. Please merge/rebase current main; I will merge this PR once the branch is updated. Combined validation before the base advanced: 248 passed, 5 optional claude_agent_sdk tests skipped, and the SQLite demo passed.