feat: implement name-based secret resolution for Lockbox secrets

[nikolaymatrosov]

This pull request adds comprehensive tests for the Lockbox secrets resolution logic and improves documentation formatting and clarity in the README.md. The most significant changes are the addition of unit tests covering various scenarios for resolving Lockbox secrets, enhancements to the Lockbox service mock for better test coverage, and updates to the documentation for improved readability and accuracy.

Testing improvements

• Added a new test suite in __tests__/lockbox-secrets.test.ts to validate the behavior of resolveLatestLockboxVersions, including cases for resolving "latest" version IDs, handling missing secrets, resolving by secret name, mixed resolution needs, and duplicate keys.
• Enhanced the Lockbox service mock in __tests__/__mocks__/@yandex-cloud/nodejs-sdk/lockbox-v1.ts by adding support for secret list management, simulating failures, and supporting secret name lookups for more realistic and flexible testing.

Documentation improvements

• Improved formatting in README.md by switching code blocks to explicit language identifiers (e.g., txt, yaml), fixing section headings, and clarifying the usage of Lockbox secrets and mounts. [1] [2] [3] [4] [5] [6]
• Updated the required IAM roles table in README.md to clarify the difference between Lockbox access by ID and by name, and when each role is required.