Acceptance tests still require SNOWFLAKE_PASSWORD

## Summary
Snowflake is deprecating password authentication sometime next year, and the Vault plugin already warns that password auth is going away. Can we update the acceptance suite? (`snowflake_test.go`) still hard-requires `SNOWFLAKE_PASSWORD` via `dsnString()` even when exercising the key-pair/JWT flows, so running `make testacc` without a password is impossible.

## Steps to Reproduce
1. Set `SNOWFLAKE_ACCOUNT`, `SNOWFLAKE_USER`, `SNOWFLAKE_DATABASE`, and `SNOWFLAKE_PRIVATE_KEY` for key-pair auth, but leave `SNOWFLAKE_PASSWORD` unset.
2. Run `VAULT_ACC=1 make testacc`.
3. The suite exits immediately because `dsnString()` returns an error about the missing password, even though the test was meant to exercise JWT auth.

## Expected Behavior
Acceptance tests should run (or skip password-only cases) when only key-pair credentials are available, so we can verify the flows that will remain once password login is removed.

## Actual Behavior
`dsnString()` returns an error and the test run aborts unless a password is configured, which forces us to keep legacy passwords purely to satisfy tests.

## Additional Notes
- `dsnString()` and other helpers gate the suite on password auth even if we only care about JWT/key-pair paths.
- Request: make password-dependent cases optional, or add parallel acceptance coverage that avoids password auth entirely.