[zhangwenjun-huawei]

Description

feat(corpus): honor A2AT_LANGUAGE for prompts and case files

• read A2AT_LANGUAGE from a2a-t-corpus/.env or the process environment, default to zh-CN, and fail fast on unsupported values
• use the configured language in the Task-T, Negotiation-T and Notification-T runtimes
• select language-specific input_case_*_.json files and skip with an actionable message when the variant is absent
• suffix non-default-language transcripts and summaries with the language
• validate language variants in the self-guards and add non-live config tests
• document the language switch in env.example and the READMEs

Related Issue(s)

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Other (describe below)

Checklist

• I have read the CONTRIBUTING guide
• All commits are signed off (git commit -s) per the DCO
• New and existing tests pass locally
• I have added tests that prove my fix is effective or my feature works (if applicable)
• I have added or updated documentation as needed
• New source files include the SPDX license header

Additional Context