Address PR #30 review comments: add validations and shared token caching

[Copilot]

Addresses review feedback from PR #30 to improve error handling, reduce redundant authentication, and enhance code quality.

Changes

• Unique entry validation: Added validate_unique_entry calls in get_vault_entry_from_path and get_vault_entry_from_type to prevent ambiguous matches when multiple entries share the same path/type

• Module-level token caching: Converted instance-based token cache to module-level shared cache, allowing secret and secret_full lookup plugins to reuse authentication tokens across instances in the same playbook run

• Entry validation: Added checks for missing entry_id and empty credential objects in get_credential, with improved error messages including server URL for debugging

• Documentation: Split long field lists in secret.py across multiple lines, removed duplicate field documentation from notes section

Token Caching Example

Before (instance-level):

# Each lookup plugin instance authenticates separately
lookup('devolutions.dvls.secret', 'db-pass')          # Auth #1
lookup('devolutions.dvls.secret_full', 'api-key')    # Auth #2

After (module-level):

# Single authentication shared across all instances
lookup('devolutions.dvls.secret', 'db-pass')          # Auth #1
lookup('devolutions.dvls.secret_full', 'api-key')    # Reuses Auth #1

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[Copilot]

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