feat(types): overridable Tailordb.Client constructor parameter

[toiroakr]

Summary

Make Tailordb.Client constructor parameter overridable through declaration merging, allowing
library consumers to customize the client configuration type while maintaining backward
compatibility.

Motivation

Previously, the Tailordb.Client constructor had a fixed parameter type with only an optional
namespace field. This design made it impossible for library consumers to:

• Override default field to narrow (e.g., make namespace "main-db" | "sub-db")

This PR introduces a flexible type system that allows consumers to extend or override the
client configuration through TypeScript's declaration merging.

Changes

Type System

• Added UserClientConfig interface: Empty by default, can be extended by consumers via
  declaration merging
• Added DefaultClientConfig interface: Contains the default namespace?: string field
• Created ClientConfig type: Merges UserClientConfig with DefaultClientConfig, where
  UserClientConfig takes precedence

Testing Infrastructure

• Set up Vitest with type checking support
• Added comprehensive type tests covering 4 scenarios:
  • default: Empty UserClientConfig (backward compatible)
  • required-field: Adding a required field (apiKey: string)
  • optional-field: Adding an optional field (timeout?: number)
  • override: Overriding default field optionality (namespace: string)

Test Configuration

• Configured Vitest with 4 separate projects for isolated type testing
• Each test scenario has its own tsconfig.json for proper type isolation

[toiroakr]

While this technically qualifies as a breaking change, because omitting it would already have caused an error, nothing actually changes in practice.