Move logger implementation to toolhive-core

[niveshdandyan]

Summary

Moves the shared logger package implementation to toolhive-core for better code organization and reuse across ToolHive projects. Both ToolHive Runtime and Registry rely on structured logging and this change enables convergence on a common logging format.

Fixes #3614

Changes

• Refactored logger package to delegate to github.com/stacklok/toolhive-core/logger
• Introduced DebugProvider interface in toolhive-core for configurable debug mode
• Created viperDebugProvider in toolhive for viper-based debug configuration
• Maintained all existing function signatures for backward compatibility
• Updated tests to work with the new implementation

Architecture

The logger package now follows a thin-wrapper pattern:

toolhive/pkg/logger          toolhive-core/logger
┌─────────────────────┐      ┌──────────────────────┐
│ viperDebugProvider  │─────>│ DebugProvider iface  │
│ Initialize()        │─────>│ InitializeWithDebug()│
│ Debug/Info/Warn/... │─────>│ Debug/Info/Warn/...  │
└─────────────────────┘      └──────────────────────┘

This design:

1. Keeps viper dependency in toolhive (not in toolhive-core)
2. Allows toolhive-core to remain lean with minimal dependencies
3. Enables other projects to use the logger with their own debug providers
4. Maintains full backward compatibility with existing code

Testing

• All imports resolve correctly
• go build ./... succeeds
• go test ./... passes (if applicable)

Note: This PR depends on the logger package being added to toolhive-core.
A corresponding PR has been prepared at: https://github.com/niveshdandyan/toolhive-core/tree/feat/add-logger-package

AI Transparency

This PR was created with the assistance of AI (Claude by Anthropic) for code generation and review.