fix(db): require explicit opt-in for destructive migration tests

[yinkev]

Summary

• remove the silent fallback from schema-destructive migration tests to the default Buzz Desktop development database;
• require both BUZZ_TEST_DESTRUCTIVE_DB=1 and an explicit non-empty BUZZ_TEST_DATABASE_URL before any test can drop the public schema;
• rename the connection helper to make its destructive contract explicit and add a pure regression for every missing/invalid gate combination.

#[ignore] was not a sufficient boundary because cargo test -p buzz-db --lib -- --ignored is a natural way to request the Postgres-backed tests. Before this change, that command could resolve to postgres://buzz:buzz_dev@localhost:5432/buzz and execute DROP SCHEMA IF EXISTS public CASCADE against a developer's live Desktop database.

Related issue

Fixes #2916

No competing implementation was found before opening this PR.

Testing

• cargo test -p buzz-db --lib — 85 passed, 121 intentionally ignored
• cargo clippy -p buzz-db --all-targets -- -D warnings
• cargo test -p buzz-db destructive_database_tests_require_explicit_opt_in_and_url --lib
• disposable PostgreSQL database:
  • migrate with cargo run -p buzz-admin -- migrate;
  • run the ordinary buzz-db package suite;
  • run the three schema-reset tests with both required environment variables and --test-threads=1 — 3 passed;
  • drop the disposable database afterward.
• git diff --check

just ci completed the workspace Rust formatting and clippy stages, then stopped at the existing file-size gate for untouched desktop/src-tauri/src/managed_agents/runtime.rs (current upstream is already above its configured ceiling). This PR does not modify that file.

[yinkev]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Nice work!

Reviewed commit: 8d691a9d5e

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