Make acceptance tests runnable with key-pair auth only

[bbhorrigan]

Summary

Snowflake is deprecating password authentication (see #172), but the acceptance test suite hard-requires SNOWFLAKE_PASSWORD via dsnString(), making the entire suite impossible to run with only key-pair credentials. This PR makes password-dependent tests optional so the suite runs with key-pair auth alone.

• Password-dependent tests skip when SNOWFLAKE_PASSWORD is unset
• Key-pair subtests in TestSnowflake_NewUser now use key-pair admin connections for initialization and cleanup
• New TestSnowflake_RenewUser_KeyPair and TestSnowflake_RevokeUser_KeyPair tests cover critical flows without password auth
• Fixes a bug in getKeyPairAuthParameters that used user instead of account in the connection URL (introduced in Add support for keypair root configuration #109), causing key-pair SQL operations to hit the wrong host
• Fixes verifyConnWithKeyPairCredential which used ParseDSN that rejects empty passwords even for JWT auth, making key-pair verification impossible

Test plan

• go vet ./... and go build ./... pass
• Unit tests pass: go test -v -run 'TestGetSnowflakeConfig|TestGetPrivateKey|TestConnectionProducer' ./...
• Full acceptance suite verified against a live Snowflake account with only key-pair env vars — password tests skip cleanly, all key-pair tests pass

Closes #172

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[bbhorrigan]

Hey, I also work at Snowflake and emailed John-Michael last week about this change. I tested this quite a bit and went through acceptance testing as requested, and found a couple of small bugs along the way.