[genai-fork] anthropic: preserve per-block thinking text on stream ca…

[orual]

…pture + outbound

The streaming capture path discarded per-block reasoning text (let _ = reasoning; at anthropic/streamer.rs) and reconstructed ThinkingBlock parts with empty text paired to real signatures. The outbound assistant-message serializer compounded this with a reasoning_texts.join("") + zip-by-index strategy that concatenated all reasoning into block[0]'s text and left block[1..] empty, scrambling the text<->signature pairing.

Anthropic validates thinking-block signatures byte-exact against the paired thinking text on replay. Both bugs silently invalidated every signature on multi-block thinking turns; the single-block case worked by coincidence because the "combined" text equalled the "per-block" text.

Fix:

• Widen StreamerCapturedData::thought_signatures (Vec) to thought_blocks: Option<Vec>. push_thought_block takes (text, sig, provenance) and preserves the pairing through reconstruction.
• Anthropic streamer pushes (reasoning, signature) together at content_block_stop instead of dropping reasoning.
• chat_stream::StreamEnd::from rebuilds ContentPart::ThinkingBlock with real per-block text. ToolCall.thought_signatures (Gemini-shaped opaque-signatures-on-tool-call semantic) is backfilled with signature strings only -- correct for Gemini's wire format which has no paired thinking text.
• Anthropic outbound (adapter_impl) walks ContentPart::ThinkingBlock parts in order and emits one {thinking, signature} per block 1:1; non-Anthropic-provenance blocks are dropped silently.
• Gemini and openai_resp streamers pass empty text via push_thought_block (their signatures are opaque with no paired text). Gemini's outbound adapter already ignores block.text and emits just {thoughtSignature: sig}, so cross-provider round-trip stays correct.

Tests:

• New tests/tests_signature_roundtrip.rs asserts inbound multi-block fixture produces two ThinkingBlocks with correct per-block text and signatures.
• New unit test in anthropic/adapter_impl replaces the previous one that codified the broken "concat into block[0], empty block[1..]" behaviour; new test asserts 1:1 text<->signature pairing on outbound.
• tests/data/yakbak/anthropic/thinking_multi_block_stream/ new fixture with two distinct thinking blocks and a trailing tool_use.

78 unit tests + yakbak (anthropic + gemini) + signature roundtrip integration pass. No behavioural changes for single-block thinking.