Fix connection hang in database_is_empty() for SerenDB/Neon targets

[taariq]

Summary

• Fixes indefinite hang when running init against SerenDB/Neon targets where the database already exists
• Reuses the existing target_client connection instead of creating a new connection in database_is_empty()
• Prevents connection pool exhaustion on serverless PostgreSQL providers

Problem

When database_is_empty() was called, it created a second database connection while the first target_client connection was still active. SerenDB/Neon has strict concurrent connection limits, and since tokio_postgres::connect has no built-in timeout, the operation would hang indefinitely waiting for a connection slot.

Reproduced on jobs:

• 438c3593-057c-45f3-9d23-6ddce6cea1bb
• 314eacf8-5fcc-48d5-822a-3350f3b5a0e3

Both hung for 6+ minutes at:

INFO: Database 'serendb' already exists on target
[HANG - no further output]

Solution

Changed database_is_empty() to accept a &Client reference instead of creating a new connection. The existing target_client is already connected to the correct database and can be reused.

Test plan

• cargo build passes
• cargo clippy passes
• cargo test passes (67 tests)
• Manual test with SerenDB target (requires deployment)

Fixes #23