[refactor] Add MockClient implementation for unit testing

[AnshulPatil2005]

PR Title format: [refactor] Add MockClient implementation for unit testing

Description

This PR adds a MockClient implementation that simulates the Extralit API behavior without external dependencies

Related Tickets & Documents

Closes #178

What type of PR is this? (check all applicable)

• Refactor
• Feature
• Bug Fix
• Optimization
• Documentation Update

Steps to QA

run tests
test_get_resources - Verify resource retrieval by name and id
test_get_resources_warnings - Verify warnings when resources not found
test_get_resource_with_missing_args - Verify errors when required args missing
test_init_with_missing_api_url - Verify error when api_url is missing
test_init_with_missing_api_key - Verify error when api_key is missing
test_workspace_create_and_list - Verify workspace CRUD operations
test_user_create_and_list - Verify user CRUD operations
test_dataset_create_and_list - Verify dataset CRUD operations
test_workspace_default - Verify default workspace retrieval
test_workspace_default_no_workspaces - Verify error when no workspaces exist
test_me_property - Verify current user property
test_resource_deletion - Verify resource deletion
test_both_name_and_id_uses_id - Verify id takes precedence over name

Added/updated tests?

• Yes
• No, and this is why: please replace this line with details on why tests
  have not been included
• I need help with writing tests

Added/updated documentations?

• Yes
• No, and this is why:
• I need help with writing docs

Checklist

• I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)