fix: ensure database directory exists before creation

[echobt]

Description

This PR fixes a bug where run_index fails if the database path resides in a non-existent directory. The fix ensures that the parent directory of the database path is created before attempting to open the database connection.

Changes

• In src/cli/commands.rs: Added logic to check and create the parent directory of the database path if it does not exist.

Reproduction

1. Configure vgrep with a db_path in a non-existent directory.
2. Run vgrep index.
3. Before fix: SQLite error about being unable to open database.
4. After fix: Directory is created and indexing proceeds.

Verification

• Added a reproduction test case (verified locally) that confirms the directory is created and database initialization succeeds.