Improve Client CLI startup performance with lazy imports

## Background

Client CLI (`backend.ai` command) startup speed is slow due to eager imports of heavy dependencies at module level. This affects developer experience and CI/CD pipeline performance.

Agent and Storage Proxy already have lazy import enforcement tests (see `tests/unit/cli/test_lazy_import_agent.py`, `test_lazy_import_storage_proxy.py`) that ensure CLI commands use lazy imports to reduce startup time.

**Related issues:** lablup/backend.ai#663, lablup/backend.ai#5327

## Current Status

### Components with lazy import tests

- ✅ Agent CLI (`ai.backend.agent.cli`)

- ✅ Storage Proxy CLI (`ai.backend.storage.cli`)

- ⚠️ Manager CLI (`ai.backend.manager.cli`) - test exists but is commented out

### Components missing lazy import tests

- ❌ \***Client CLI**\* (`ai.backend.client.cli`) - large module with ~50 command files

## Goals

1. Add lazy import enforcement test for Client CLI

2. Refactor Client CLI imports to use lazy loading (imports inside functions)

3. Review and potentially re-enable Manager CLI lazy import test

## Tasks

### 1. Add test for Client CLI

- Create `tests/unit/cli/test_lazy_import_client.py`
- Follow the pattern from agent/storage tests
- Identify heavy modules that should not be loaded at CLI startup (e.g., session logic, API models, pandas/numpy dependencies)

### 2. Refactor Client CLI imports

- Move heavy imports inside command functions
- Ensure CLI help text remains fast
- Verify startup time improvement

### 3. Review Manager CLI test

- Investigate why `test_lazy_import_manager.py` is commented out
- Fix any issues and re-enable if possible

## Success Criteria

- Client CLI startup time reduced significantly
- All lazy import tests passing
- No regression in CLI functionality

JIRA Issue: BA-4920