Refactor: Separate OAuth logic from MCP Server

[rnovatorov]

Moved OAuth and authentication logic from src/enapter_mcp_server/mcp/server.py to a new module src/enapter_mcp_server/mcp/auth.py.
Introduced AuthProvider, HeaderAuthProvider, and OAuthProxyAuthProvider classes to handle authentication strategies.
Updated Server class to use AuthProvider for configuring FastMCP authentication and retrieving Enapter API authentication.
This change improves modularity and testability by decoupling authentication logic from the main server implementation.

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PR created automatically by Jules for task 13770356607907446481 started by @rnovatorov

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