fix(anthropic): support code execution tool results

[gold-silver-copper]

Summary

• add explicit support for Anthropic code_execution_tool_result content blocks
• preserve normal and encrypted code-execution results through non-streaming, streaming, and multi-turn transcript round-trips
• add focused unit coverage and enable a live-recorded cassette regression for web_search_20260209

Closes #2154.

Root cause

Anthropic's web_search_20260209 tool uses hosted code execution for dynamic filtering. Responses can therefore include top-level code_execution_tool_result blocks containing either code_execution_result or encrypted_code_execution_result payloads.

Rig's tagged Anthropic Content enum did not recognize that top-level variant, causing response deserialization to fail before callers received the completion.

Implementation

• model code_execution_tool_result explicitly with its tool_use_id and provider-native JSON content
• preserve the complete block using the existing anthropic_content metadata convention
• accept preserved blocks when converting Rig messages back to Anthropic requests
• handle the block consistently in streaming content_block_start events
• recognize code-execution results when evaluating server-tool transcript boundaries

The implementation deliberately avoids a catch-all unknown-content fallback so unrelated future Anthropic variants still fail visibly instead of being silently discarded.

Regression coverage

The cassette was recorded against Anthropic with web_search_20260209 and contains both normal and encrypted code-execution results. The enabled regression test verifies that the request succeeds and that a code_execution_tool_result block remains available in the normalized response metadata.

Focused unit tests cover:

• normal code_execution_result deserialization
• encrypted encrypted_code_execution_result deserialization
• non-streaming raw metadata preservation and request round-trip
• streaming block preservation

Verification

• cargo fmt --check
• git diff --check
• cargo test -p rig-core code_execution_tool_result -- --nocapture — 3 passed
• env -u ANTHROPIC_API_KEY cargo test -p rig --all-features --test anthropic web_search_with_dynamic_filtering_succeeds -- --nocapture --test-threads=1 — passed
• cargo test -p rig --all-features --test anthropic cassette_safety -- --nocapture --test-threads=1 — 2 passed
• env -u ANTHROPIC_API_KEY cargo test -p rig --all-features --test anthropic anthropic::cassette -- --nocapture --test-threads=1 — 66 passed
• cargo clippy --all-targets --all-features -- -D warnings — passed
• cargo test — passed

All required GitHub checks pass: formatting, WASM check, clippy, tests, documentation, and doctests.

Review

An independent review of the complete diff found no P0/P1 or lower-severity correctness or security issues. Cassette safety checks pass, the fixture contains no credentials, and there are no unresolved review threads.