fix: handle named custom providers and Z.AI overload retries

[benbenlijie]

Summary

• Match custom:<provider_key> / custom:<name> entries when merging custom provider extra_body
• Add narrow adaptive backoff for Z.AI Coding Plan GLM-5.2 transient overload 429s (code 1305 / temporarily overloaded)
• Surface long adaptive waits immediately so the TUI/CLI does not appear frozen during multi-minute overload backoff

Test Plan

• python -m pytest tests/test_retry_utils.py tests/agent/test_custom_provider_extra_body.py -q -o 'addopts='

[benbenlijie]

It looks like this patch has landed on main as equivalent upstream commit 6f2b2a1f34 (fix: handle named custom providers and Z.AI overload retries).\n\nI verified with git cherry -v origin/main fix/named-custom-zai-retry, which marks this PR commit as already absorbed. Closing this PR as superseded by the upstream main commit. Thanks!