Migrate connection string to new connections model

[kristof-siket]

Summary

Migrates create-db from reading deprecated apiKeys[0].directConnection fields to the new connections[0].endpoints.direct.connectionString from the Management API response (PTL-1046).

• Reads the connection string directly from connections[0].endpoints.direct.connectionString, falling back to pooled.connectionString
• Preserves backward compatibility by falling back to the legacy apiKeys path when connections is absent (for older API versions)
• Removes manual postgresql://... DSN construction from the main flow
• Adds ConnectionEndpoint and Connection type interfaces reflecting the new response shape

Context

PR #3365 added the new connection model to the Management API. The deprecated fields (apiKeys[0].directConnection.*) will be removed in a future release — this migration must land before that happens.

Test plan

• Run npm test (unit tests pass locally)
• Manually test against production Management API — verify connection string is returned correctly
• Manually test against preview Management API — verify new connections[] path is used
• Verify backward compat: when API returns only apiKeys (no connections), the legacy fallback still produces a valid connection string

Closes PTL-1046

Summary by CodeRabbit

• New Features

  • Support for multiple connection endpoint types (direct, pooled, accelerate) for more flexible connectivity.

• Improvements

  • Connection string resolution now prefers endpoint-provided connection strings instead of constructing them from individual credentials.
  • Internal flow refined for more reliable detection of available endpoints; no public API changes.