fix(gateway): preserve rate-limit failure metadata

[AtakanGs]

Preserves failed and failure_reason metadata when a gateway agent run
returns a non-empty rate-limit error response.

On retry exhaustion, run_conversation() already returns failure metadata,
but GatewayRunner._run_agent_inner() dropped it on the non-empty
final_response path. The outer gateway could therefore treat the generated
429 text as a normal assistant response and persist it into the session
transcript.

This change forwards the failure metadata through that return path, allowing
the existing transient-failure handling to preserve the user turn without
storing the gateway-generated error as assistant output.

Regression coverage verifies that:

• a non-empty 429 response retains its failure metadata;
• the 429 text is not persisted as an assistant transcript entry;
• the same session can switch to another provider and continue successfully
  without replaying the previous 429 error.

The Desktop/TUI route was reviewed separately. Current main already keeps
retry-exhaustion error text out of backend conversation history and rebuilds
the runtime during a provider switch, so this PR does not modify that path.

Related Issue

Closes #64446

Testing

• 19 passed
• Ruff checks passed
• git diff --check passed

[tonydwb]

Code Review Summary

Verdict: Approved

Overview

Gateway fix: preserves rate-limit failure metadata instead of dropping it, so callers can distinguish rate-limit errors from other failures. 226 additions.

Assessment

• Correctness: Preserving rate-limit metadata is the right behavior for proper error handling downstream.
• Security: No security-sensitive changes.
• Debug artifacts: None.

Summary

Clean fix. LGTM.

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Reviewed by Hermes Agent

[teknium1]

Thanks for the focused regression fix. The premise is confirmed on current main: gateway/run.py:19672 rebuilds the non-empty result without failed, while gateway/run.py:12224 only suppresses assistant-error transcript persistence when that flag reaches the outer handler. The change in 68766bf88450645de60089b9f3a309f38a3ba0a6 restores that contract and preserves the classified metadata emitted by agent/conversation_loop.py:4201-4208.

Automated hermes-sweeper review.