fix: validate extended thinking blocks and tool use/result pairing

.changeset/fix-extended-thinking-blocks.md

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+---
+"@centralinc/browseragent": patch
+---
+
+Fix critical message history management issues preventing 400 errors
+
+**Two Major Fixes:**
+
+1. **Extended Thinking Block Validation**: When `thinkingBudget` is enabled, the API requires every assistant message to start with a thinking or redacted_thinking block. Added `ensureThinkingBlocksForExtendedThinking()` to filter out assistant messages without thinking blocks and their corresponding user messages to maintain conversation flow.
+
+2. **Tool Use/Result Pairing**: Fixed "unexpected tool_use_id found in tool_result blocks" error. The API requires each tool_result to have its corresponding tool_use in the IMMEDIATELY PREVIOUS message, not just anywhere in history. Rewrote `cleanMessageHistory()` to validate pairing on a per-message basis.
+
+**Errors Fixed:**
+- `"Expected thinking or redacted_thinking, but found text"`
+- `"unexpected tool_use_id found in tool_result blocks: [id]. Each tool_result block must have a corresponding tool_use block in the previous message"`
+
+**Testing:** Extended thinking test passes with 15+ tool calls across multiple turns without errors.