chore: add some extra log level validations

[nirinchev]

Proposed changes

This is a follow-up on #486 - it adds an extra test to ensure our log levels array is exhaustive and a parameter validation for the http endpoint.

[Copilot]

Pull Request Overview

This PR adds validation and testing improvements for log levels in the MongoDB MCP server. It ensures the log levels array is complete and validates log level parameters in HTTP endpoints.

• Added a test to verify the McpLogger.LOG_LEVELS array contains all possible log levels from the MCP SDK schema
• Added parameter validation to the SetLevelRequestSchema handler to reject invalid log levels
• Made the LOG_LEVELS array public and added as const assertion for better type safety

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
tests/unit/logger.test.ts	Added test to validate log levels array completeness against MCP SDK schema
src/server.ts	Added validation for log level parameter in SetLevelRequestSchema handler
src/common/logger.ts	Changed LOG_LEVELS from private to public and added const assertion

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