[codex] fix(fallback): honor explicit api_mode

[kohoj]

Fixes #57582.

Problem

The original fallback-index report was corrected in the issue thread: the live failure was that a fallback_providers entry could set api_mode: anthropic_messages, but fallback activation ignored that field and inferred transport only from provider name, URL, and model. A custom Anthropic-compatible proxy without an api.anthropic.com host or /anthropic suffix defaulted to chat completions and posted to the wrong endpoint.

Root cause

try_activate_fallback() read provider/model/base_url/api_key from each fallback entry but never read fb["api_mode"]. The primary runtime path supports explicit api_mode; fallback activation did not.

Fix

• Add an explicit fallback api_mode gate for supported transports before the existing inference chain.
• Leave existing provider/URL/model inference intact when api_mode is absent.
• Warn and fall back to inference for unsupported api_mode values.

Tests

• $HOME/.hermes/hermes-agent/venv/bin/python -m pytest tests/run_agent/test_provider_fallback.py -q
• $HOME/.hermes/hermes-agent/venv/bin/python -m ruff check agent/chat_completion_helpers.py tests/run_agent/test_provider_fallback.py

[alt-glitch]

This was generated by AI during triage.

Duplicate of #16346 (earliest open canonical, fix(agent): respect fallback api_mode overrides). Same mechanism -- honor an explicit api_mode on a fallback entry before URL/provider inference in try_activate_fallback() (relocated from run_agent.py to agent/chat_completion_helpers.py). #29749 already dedups into #16346. Note: issue #57582's root cause was corrected in -thread to the fallback-index-reset bug, which #57700 fixes via a DIFFERENT code path (agent_runtime_helpers.py) -- related, not the same fix.

[kohoj]

Closing this after re-reading the duplicate feedback: #16346 is the earlier canonical PR for the explicit fallback api_mode path, and the corrected #57582 thread points at a different fallback-index-reset root cause covered elsewhere. I won't keep a non-canonical competing fix open.