fix(models): accept opus-4.8 and hoist inline system messages

[sour2001]

What & Why

Two related fixes so claude-opus-4.8 works end-to-end through the gateway. Both surfaced as 422 errors when pointing a client (Claude Code) at the Anthropic endpoint.

1. Add claude-opus-4.8 to FALLBACK_MODELS

The fallback model list topped out at claude-opus-4.7. When Kiro's /ListAvailableModels doesn't return 4.8 (or the gateway falls back to the static list), the resolver treated it as an unknown pass-through model, which Kiro then rejected. Added the ID so it resolves from cache like the other current models. The name normalizer already handles the common input shapes (claude-opus-4-8, claude-4.8-opus-high, date suffixes) — they all map to claude-opus-4.8.

2. Hoist inline role: "system" messages

Anthropic's schema only accepts user/assistant inside the messages array; the system prompt belongs in the top-level system field. Some clients (Claude Code, LangChain, OpenAI-style callers) inline role: "system" messages instead, which failed with:

422 literal_error: Input should be 'user' or 'assistant' ... input: "system"

Added a mode="before" validator on AnthropicMessagesRequest and AnthropicCountTokensRequest that moves inline system messages into the top-level system field before per-message validation runs. It:

• Merges with an existing top-level system (string or list-of-blocks form) rather than clobbering it
• Joins multiple inline system messages in order
• Handles system content given as a string or as a list of text blocks
• Strips empty inline system messages (which would otherwise still 422)
• Is a no-op when there are no inline system messages

Applied to both request models for consistency (messages + count_tokens).

Test coverage

All new tests follow the existing Arrange-Act-Assert + print-diagnostics style.

• tests/unit/test_models_anthropic.py::TestInlineSystemMessageHoisting — 10 tests: single/multiple inline system messages, merge with existing string/list system, block-form content, empty content, count_tokens endpoint, and typed-object passthrough.
• tests/unit/test_config.py::TestFallbackModelsIntegration::test_opus_4_8_present_and_resolves — verifies claude-opus-4.8 is in the list and resolves from cache for both dot and dash client formats.

Full suite: 1702 passed.

[cla-bot]

Thanks for the PR! 🎉

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[sour2001]

I have read the CLA and I accept its terms

[musa-2066462]

I just tested this code locally and it works perfectly.
U just need to change the anthropic model to this :
export ANTHROPIC_MODEL="claude-opus-4-8-20260528"

[ankitcharolia]

@sour2001 @musa-2066462 could you give a try to this gateway: https://github.com/ankitcharolia/kiro-gateway

It supports all necessary endpoints for OpenAI and Anthropic.