feat: add read commands for plans, suites, and cases (#401)

[cosmicdreams]

Issue being resolved: #401

Solution description

Added five read commands that fetch TestRail data as JSON to stdout, using the
existing cmd_*.py auto-discovery pattern. get_plans and get_plan are
exposed as public methods on ApiRequestHandler with caching (matching the
existing get_suites pattern). get_suites, get_cases, and get_case call
APIClient.send_get directly. All commands honour global auth flags and env vars.

Changes

• trcli/api/api_request_handler.py — new get_plans(project_id) and
  get_plan(plan_id) public methods + private __get_all_plans with caching
• trcli/commands/cmd_get_plans.py — list all plans for a project
• trcli/commands/cmd_get_plan.py — fetch a single plan by ID
• trcli/commands/cmd_get_suites.py — list all suites for a project
• trcli/commands/cmd_get_cases.py — list cases with pagination + optional --section-id filter
• trcli/commands/cmd_get_case.py — fetch a single case by ID
• tests/test_cmd_get_*.py — 30 new CLI tests (happy path, error path, missing args)
• tests/test_api_request_handler.py — 4 new handler unit tests
• README.md — new "Reading data from TestRail" section with usage table and examples
• CHANGELOG.MD — [Unreleased] entry

Potential impacts

• No changes to existing commands or cli.py — new files only, plus additive
  changes to api_request_handler.py
• New commands appear in trcli --help automatically via folder scan
• Caching in get_plans uses the same _cache mechanism as get_suites —
  no new cache behaviour introduced

Steps to test

1. trcli -h <host> -u <user> -k <key> get_plans --project-id 1 → JSON array of plans
2. trcli -h <host> -u <user> -k <key> get_plan --plan-id 42 → JSON plan object
3. trcli -h <host> -u <user> -k <key> get_suites --project-id 1 → JSON array of suites
4. trcli -h <host> -u <user> -k <key> get_cases --project-id 1 --suite-id 3 → JSON array of cases
5. trcli -h <host> -u <user> -k <key> get_cases --project-id 1 --suite-id 3 --section-id 10 → filtered
   cases
6. trcli -h <host> -u <user> -k <key> get_case --case-id 5001 → JSON case object
7. python3 -m pytest -c ./tests/pytest.ini -W ignore::pytest.PytestCollectionWarning tests/test_cmd_get_*.py tests/test_api_request_handler.py → all pass

PR Tasks

• PR reference added to issue
• README updated
• Unit tests added/updated

[acuanico-tr-galt]

Hi @cosmicdreams, thank you for submitting this PR. Once ready, can you point this to release/1.13.4 instead of main? We'll be reviewing the changes before we can merge. Thank you.