Summary

Fixes ROO-312

When extended thinking is enabled, Anthropic's API returns signature_delta events that contain cryptographic signatures for thinking blocks. These signatures are required when passing thinking blocks back to the API for tool use continuations.

Reference: Anthropic Extended Thinking Documentation

The Problem

Without the signature, thinking blocks were stored as generic 'reasoning' blocks which get filtered out by anthropic-filter.ts, causing the API validation error:

Expected thinking or redacted_thinking, but found tool_use

Before (Bug)

1. API streams thinking block + signature_delta ← signature IGNORED
2. getThoughtSignature() returns undefined
3. Thinking stored as: { type: "reasoning", text: "..." }
4. anthropic-filter.ts filters out "reasoning" blocks  
5. API sees: [tool_use block without thinking] → ERROR

After (Fixed)

1. API streams thinking block + signature_delta ← signature CAPTURED  
2. getThoughtSignature() returns "sig_abc123"
3. Thinking stored as: { type: "thinking", thinking: "...", signature: "sig_abc123" }
4. anthropic-filter.ts passes "thinking" blocks through
5. API sees: [thinking block with signature, then tool_use] → SUCCESS

Relevant Documentation Sections

From Anthropic Extended Thinking Docs:

1. Streaming signature format:

event: content_block_delta
data: {"type": "content_block_delta", "index": 0, "delta": {"type": "signature_delta", "signature": "EqQBCgIYAhIM..."}}

2. Thinking block format with signature:

{
  "type": "thinking",
  "thinking": "Let me analyze this step by step...",
  "signature": "WaUjzkypQ2mUEVM36O2TxuC06KN8xyfbJwyem2dw3URve/op91XWHOEBLLqIOMfFG/UvLEczmEsUjavL...."
}

3. Signature purpose: "Full thinking content is encrypted and returned in the signature field. This field is used to verify that thinking blocks were generated by Claude when passed back to the API."

Changes

• Add lastThinkingSignature property to store captured signature
• Add getThoughtSignature() method to expose signature to Task.ts
• Handle signature_delta events in content_block_delta
• Emit thinking_complete chunk when thinking block finishes with signature
• Add comprehensive tests for signature capture functionality

Testing

• All 34 Anthropic provider tests pass
• Added 5 new tests for signature capture scenarios