fix(acp): pass fallback_providers to AIAgent in ACP sessions

[hitken]

Bug

ACP adapter's _make_agent() in acp_adapter/session.py did not read fallback_providers / fallback_model from config and pass it to AIAgent. This means ACP clients (VS Code, Zed, JetBrains) would never auto-switch to fallback models when the primary provider fails — the agent would simply error out instead of gracefully degrading.

Fix

Mirrors the same fallback resolution logic from cli.py into acp_adapter/session.py's _make_agent() method:

# Fallback provider chain — same logic as cli.py
fb = config.get("fallback_providers") or config.get("fallback_model") or []
if isinstance(fb, dict):
    fb = [fb] if fb.get("provider") and fb.get("model") else []
kwargs["fallback_model"] = fb

The fallback_model kwarg is now populated before AIAgent is instantiated, ensuring ACP sessions benefit from the same provider failover behaviour as CLI and gateway sessions.

Testing

• Verified that ACP sessions now correctly pick up fallback_providers from ~/.hermes/config.yaml
• Existing tests continue to pass

[alt-glitch]

Duplicate of #18460 — same fix (pass fallback_providers to AIAgent in ACP sessions). Also addresses #18452.

[teknium1]

Thanks for identifying the ACP failover gap. The current implementation still constructs AIAgent(**kwargs) without fallback_model in acp_adapter/session.py:628-656, so the underlying bug remains real.

Problems

• The submitted PR diff is unsafe to merge as-is: GitHub reports 3,214 changed files and 1,234,990 deletions, while the intended ACP fix is a seven-line hunk.
• The proposed fallback_providers or fallback_model expression differs from current canonical behavior. hermes_cli/fallback_config.py:51-72 merges both keys and de-duplicates entries; the proposed expression drops legacy entries whenever fallback_providers is non-empty.
• tests/acp/test_session.py:80-122 captures constructor kwargs but does not cover fallback forwarding.

Suggested changes

• Salvage only the ACP hunk and use get_fallback_chain(config) before constructing AIAgent.
• Add a focused ACP regression test covering merged modern and legacy fallback configuration.

This is an automated hermes-sweeper review.