[BUG] save_data silently overwrites existing files #539

Severity: P2. Verified live 2026-08-08.

Problem

save_data silently overwrites an existing file with no indication.

save_data(data_handle=<20-row handle>, path="out.csv", format="csv")  -> {"success": true, "rows": 20}
save_data(data_handle=<3-row handle>,  path="out.csv", format="csv")  -> {"success": true, "rows": 3}

The second call replaces the file with no overwritten flag or warning. This is inconsistent with save_model, which refuses to write into a non-empty path.

Proposed fix

Return overwritten: true (or warn), or require an explicit overwrite=true opt-in — matching save_model's refuse-on-non-empty behavior.

(Catalogued as NB-12 in MCP_TEST_FINDINGS.md.)