fix(sse): relocate directive-only messages off messages[0]

[HouMinXi]

Problem: Anthropic rejects Claude Code-style directive messages when they land at messages[0]. Measured in production on 2026-08-15: 122x 400 in a single hour on the offical-claude combo (fleet-clients key, claude-opus-5), all with the same upstream error:

messages.0: use the top-level 'system' parameter for the initial system prompt;
the directive-only form (content: [] with output_config) is accepted at any position

Claude Code 2.1.154+ clients send directives as system-role messages with an empty content array and a message-level output_config. The mid-conversation-system passthrough (provider === "claude" + 1M-context beta models, shouldUseMidConversationSystem) deliberately keeps system-role messages inside messages[], so a directive that arrived first went upstream unchanged and 400'd. The other passthrough path (extractSystemRoleMessages) lifted the message away but silently dropped the directive's output_config, losing the client's output-format configuration.

Fixes:

1. open-sse/handlers/chatCore/claudeSystemRole.ts — new relocateDirectiveOnlyMessages(): collects the whole leading run of empty system/developer messages and relocates the directive-only ones (empty content array + message-level output_config) past the first real (user/assistant) turn, in order, where Anthropic accepts the form ("accepted at any position"). Plain empty system messages carry nothing and are dropped. When the conversation has no real turn, the first directive's output_config is folded into the top-level parameter (an explicit top-level value wins) and the run is dropped.
2. extractSystemRoleMessages() — no longer silently discards a directive's message-level output_config; it is folded into the top-level parameter whether or not the message also carried text blocks (an explicit top-level value wins; among several directives the first wins).
3. open-sse/handlers/chatCore.ts — the mid-conversation-system passthrough now calls relocateDirectiveOnlyMessages().

Verification: new tests/unit/claude-directive-only-relocation.test.ts (16 tests: relocation, consecutive directives, empty-system+directive runs, walking past text system messages, developer-role directives, the no-real-turn fallback, top-level precedence, no-op cases) and tests/unit/claude-directive-midconv-passthrough.test.ts (a handleChatCore integration test with a mocked fetch asserting the directive never reaches upstream at messages[0]). Bug-injection proven at each layer: disabling the extract-fold made the preserve test fail; disabling the relocation made the move tests fail; removing the chatCore call made the integration test fail; reverting restores green. Regression suites: system-role-extraction, claude-system-role-cache-boundary, g13-combo-chatcore-golden, agentrouter-chatcore-protocols, claude-code-compatible-request, claude-code-compatible-helpers, cc-compatible-provider, claude-helper-minimax-output-config — 104/104. npm run typecheck:core clean, ESLint clean on all touched files.

⚠️ base-red inherited: base tip CI has CodeQL and Build Docker (linux/arm64, linux/amd64) failing on the upstream tree itself (run https://github.com/diegosouzapw/OmniRoute/actions/runs/31858514611) — unrelated to this change.

[diegosouzapw]

Thanks @HouMinXi — this is a genuine 400 regression (122/hour on the Claude combo) and the callback is thorough: the relocation handles leading runs, null-safety, developer-role directives, and the no-real-turn fold to top-level, all backed by 12 unit tests plus an integration pass-through. Excellent work. Note the red CI is an inherited base-red (hung unit suite), not this change. 🚀