fix: use attribute access for AuthConfig instead of dictionary access

[wcollins]

📒 Description

Fixes server startup failure in v0.11.0 caused by auth.py using dictionary-style access on AuthConfig after the config module was refactored to use frozen Pydantic dataclasses.

Fixes #296

🔍 Type of Change

• Bug fix

🔧 Changes Made

• Updated src/itential_mcp/server/auth.py to use attribute access instead of .get() and [] dictionary access
• Changed type hints from dict[str, Any] to AuthConfig
• Fixed field name mappings (e.g., redirect_uri → oauth_redirect_uri)
• Updated tests to use AuthConfig dataclass instances

🧪 Testing

• All 2069 tests pass
• Ruff linter passes with no issues
• Verified attribute access works correctly for all auth types (none, jwt, oauth, oauth_proxy)

📋 Checklist

• Code follows the project's style guidelines
• Self-review of code has been performed
• Commits follow conventional format (type: subject)
• No secrets or credentials committed