test(sandbox): cover default template listing

[miclle]

Summary

• add success, API error, and camelCase alias coverage for list_default_templates
• add a live integration test for the default-template endpoint
• add a read-only default-template example and extend the existing template example

Background

SandboxClient.list_default_templates() already exposes GET /default-templates, but the public behavior lacked direct unit coverage, live integration validation, and a focused read-only example. This aligns the Python SDK verification surface with qiniu/go-sdk#230.

Validation

• flake8 --show-source --max-line-length=160 ./qiniu
• flake8 --show-source --max-line-length=160 on the changed examples and tests
• Sandbox unit tests: 147 passed
• CI public test selection with the mock server: 215 passed
• root .env live integration test: passed
• root .env read-only example: returned 8 default templates
• git diff --check

[fennoai]

No issues found in the changed tests and examples. The new unit coverage matches the existing SandboxClient.list_default_templates() behavior, and the integration/example additions are consistent with nearby sandbox conventions.

Validation: changed files compile successfully with python -m py_compile, and git diff --check passes. The focused pytest suite was not run because pytest is not installed in this workspace.