[Codex] 1.1: Add SerenDB API data structures

[taariq]

Parent: #28

Task

Add new data structures to src/serendb/client.rs for API responses.

File to Modify

src/serendb/client.rs

What to Add

Add these structs after the existing Project struct: Branch, Database, ConnectionStringResponse, CreateDatabaseRequest, PaginatedResponse, Pagination.

See docs/plans/api-key-flow-implementation.md Task 1.1 for full code.

Tests to Write

• test_branch_deserialization
• test_database_deserialization

How to Test

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

Commit Message

feat(serendb): Add data structures for branches and databases

[taariq]

Implemented the new SerenDB API structs (Branch, Database, ConnectionStringResponse, CreateDatabaseRequest, PaginatedResponse, Pagination) plus the deserialization tests outlined in Task 1.1. Verified with cargo test serendb::client::tests -- --nocapture.