fix(bedrock): preserve adaptive-thinking signatures in streaming reasoning

[byQuexo]

Closes #1684.

Summary

Streaming counterpart to #1675. The non-streaming assistant_content.rs empty-text guard fixed in that PR has an analogue in streaming.rs: the ContentBlockStop handler skips emitting RawStreamingChoice::Reasoning whenever reasoning_state.content.is_empty(), even if a Signature delta has populated reasoning_state.signature. Bedrock adaptive thinking can produce exactly that shape — ReasoningContentBlockDelta::Signature with no preceding non-empty Text delta — and the signature is silently dropped.

Symptom

In a multi-turn streaming conversation against eu.anthropic.claude-opus-4-7 (or any Bedrock Anthropic model with adaptive thinking enabled), the consumer never receives a RawStreamingChoice::Reasoning { signature: Some(_), .. } event for signature-only blocks. On the next turn, replaying that conversation back to Bedrock fails with:

messages.N.content.0.thinking.signature: Field required

— the same error that motivated #1675 on the non-streaming path.

Fix

Extracted a finalize_reasoning(state) helper from the inline emission logic and relaxed the gate so a Reasoning chunk is emitted whenever either content or signature is present; both-empty is still skipped.

fn finalize_reasoning(
    state: ReasoningState,
) -> Option<RawStreamingChoice<BedrockStreamingResponse>> {
    if state.content.is_empty() && state.signature.is_none() {
        return None;
    }
    Some(RawStreamingChoice::Reasoning { ... })
}

The helper extraction makes the emission decision unit-testable — the original inline form had no driver for the stream! block. Four cases are now covered:

• content + signature → emits Text block with both
• signature only → emits Text block with empty text + signature (the bug fix)
• content only → emits Text block with content + no signature
• both empty → emits nothing (preserves prior behavior)

Verification

Confirmed end-to-end against real Bedrock with eu.anthropic.claude-opus-4-7 and thinking={type: adaptive}, output_config={effort: high}:

[smoke] Reasoning emitted: text_len=0, sig_prefix=Er0OClkIDRAB
[smoke] totals: reasoning_chunks=1, text_chunks=0
test streaming_emits_reasoning_signature_for_adaptive_thinking ... ok

text_len=0 with a populated signature is the exact shape that pre-fix would have been dropped at the !content.is_empty() guard.

Test plan

• cargo test -p rig-bedrock --lib (70 + 4 new tests pass)
• cargo clippy -p rig-bedrock --tests -- -D warnings
• cargo fmt -p rig-bedrock --check
• Live verification against Bedrock Opus 4.7 with adaptive thinking (output above)

Notes

• Branch is based on main post-fix(bedrock): handle adaptive-thinking interactions for prompt caching and reasoning conversion #1675, so this PR contains only the streaming-side change.
• Non-streaming Bedrock (already shipped in fix(bedrock): handle adaptive-thinking interactions for prompt caching and reasoning conversion #1675) is unaffected — different code path.
• No public API change; finalize_reasoning is private to the module.