Memory Search Schema 增加 Mistral provider 兼容（PR #14934）

• 你的配置包含 memory_search 且可能使用 Mistral 作为主或 fallback provider。
• 可执行最小回归：同一查询在更新前后进行对比。

1. Capture a failing or flaky memory_search case involving Mistral provider configuration.
2. Upgrade to PR #14934 build and restart gateway.
3. Re-run identical memory_search query and compare schema validation/fallback behavior.
4. Test both primary and forced-fallback paths to ensure consistent result format.
5. Document provider matrix in runbook (openai / anthropic / mistral / fallback order).

• This patch addresses schema compatibility; retrieval quality still depends on embeddings/index settings.
• Provider naming/alias differences across versions may require config audit（需验证）。

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