Address code review feedback: fix typos, entity lifecycle, and remove redundant code

[Copilot]

Addresses all actionable review comments from the initial PR review.

Entity Lifecycle

• Use @PrePersist to initialize createdAt at persistence time instead of instance creation
• Add @Builder.Default to deleted field for proper builder pattern integration

@Column(name = "created_at", nullable = false)
private java.time.Instant createdAt;

@PrePersist
protected void onCreate() {
    this.createdAt = java.time.Instant.now();
}

Service Layer

• Remove duplicate validation logic already handled by DTO @NotBlank annotations
• Remove corresponding tests for null/empty/blank title (validation now at DTO boundary)

Code Quality

• Fix spelling: 'enity' → 'entity', 'names' → 'named'
• Remove accidental IDE error comments from BookService.java and BookRepository.java
• Remove 105+ lines of commented-out test code
• Remove Spring Boot config from openapi.yml (OpenAPI spec only)

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.