fix: preserve conversation context when provider fallback activates

[rfpassos]

Summary

• rebuild api_messages on every retry attempt instead of reusing a payload prepared for the previous provider
• prevent provider-specific transforms (for example Anthropic/OpenRouter prompt caching wrappers) from leaking into fallback requests
• add regression coverage for fallback from codex/chat paths and prompt-cached Claude requests
• make fallback tests self-contained so they do not depend on local provider config

Root cause

When the primary model failed, Hermes switched provider / model / api_mode in-place, but continued using an api_messages payload that had already been prepared for the previous backend.

That meant fallback requests could inherit backend-specific payload mutations such as:

• Anthropic/OpenRouter cache_control wrappers
• provider-specific tool-call sanitization state
• message formatting intended for the old API mode

In practice this could look like context loss or broken continuity after fallback, because the new backend was not receiving a fresh request built from the canonical conversation state.

Fix

• extracted per-attempt payload construction into AIAgent._build_api_messages_for_attempt(...)
• moved API message reconstruction inside the retry loop
• ensured each retry/fallback attempt recalculates request size from the rebuilt payload
• preserved newer upstream behavior while resolving the cherry-pick conflict (/steer pre-API drain, cache layout selection, tool-argument repair)

Tests

Added regression coverage in:

• tests/run_agent/test_fallback_context_preservation.py

Validated with:

• pytest tests/run_agent/test_fallback_context_preservation.py -q
• pytest tests/run_agent/test_fallback_model.py -q
• pytest tests/run_agent/test_run_agent_codex_responses.py -q

Why this matters

Fallback should always start from the canonical conversation history, not from a payload already transformed for a different provider. This keeps tool state, prompt context, and conversation continuity intact when Hermes has to fail over mid-turn.

[alt-glitch]

Likely duplicate of PR #13654 — same root cause: fallback reuses provider-specific payload instead of rebuilding from canonical conversation state.