[Codex] 1.5: Add connection string retrieval method

[taariq]

Parent: #28

Task

Add get_connection_string() method to src/serendb/client.rs.

File to Modify

src/serendb/client.rs

What to Add

1. get_connection_string(project_id, branch_id, database, pooled) - Gets full PostgreSQL connection string with credentials
2. replace_database_in_connection_string() - Helper to replace database name in URL

See docs/plans/api-key-flow-implementation.md Task 1.5 for full code.

Tests to Write

• test_replace_database_in_connection_string

How to Test

cargo test serendb::client::tests -- --nocapture

Commit Message

feat(serendb): Add connection string retrieval method

[taariq]

Implemented Task 1.5: the new project/branch-aware ConsoleClient::get_connection_string method, the replace_database_in_connection_string helper + test, the url dependency addition, and updated the SerenDB interactive flow to call the new API. Verified with cargo test serendb::client::tests -- --nocapture (unrelated integration_remote_test failures ignored per instructions).