Respect explicit api_mode on fallback providers

[sunnysabor]

Summary

Preserve explicit api_mode on fallback provider entries.

A fallback entry like:

fallback_providers:
- provider: custom
  model: gpt-5.4
  api_mode: chat_completions

should continue using chat_completions after fallback activation. It should not be auto-upgraded to codex_responses based only on the GPT-5-style model name.

## Motivation

Current main respects explicit api_mode for the primary model, but fallback activation can still recompute the transport from provider/model heuristics and ignore the fallback entry-level override.

This is related to #29749 and the custom GPT-5 api_mode issues discussed around #10473 / #10548.

## Changes

- Respect explicit fallback entry api_mode before applying provider/model heuristics.
- Add a regression test for a custom GPT-5-style fallback with explicit api_mode: chat_completions.

## Test plan

scripts/run_tests.sh tests/run_agent/test_provider_fallback.py

Result:

23 passed

[alt-glitch]

Duplicate of #24631 and #16346 — same fix: respect explicit api_mode on fallback_providers entries over provider/model heuristics. #16346 is the canonical (earliest open).

[sunnysabor]

Thanks for the clarification. Closing this as a duplicate of #16346.