fix(agentic): preserve only unparseable tool arguments - #1951

Summary

• Return provided_arguments only when a provider tool call could not be parsed and retain the complete raw payload.
• Stop echoing already-parsed arguments for validation errors.
• Cover both full raw-argument preservation and parsed-validation omission.

Type and Areas

Type:

Bug fix

Areas:

Rust core, tool contracts

Motivation / Impact

Parsed tool arguments are already available in the preceding assistant tool call, so repeating them in an error result wastes context and can duplicate sensitive input.

For malformed argument JSON, truncating the raw payload can hide the actual parse failure and resemble output truncation. The error path now preserves the complete raw provider payload only when no structured argument representation is available.

Verification

• pnpm run fmt:rs
• cargo test -p bitfun-agent-tools --test tool_contracts — 104 passed
• cargo test -p bitfun-core error_result_ --lib — 2 passed
• git diff --check

Reviewer Notes

• provided_arguments is emitted only when ToolCall.is_error and raw_arguments is present.
• No migration, user-visible copy, locale, or manual UI verification is required.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above.
• User-facing strings, docs, and locales are unchanged.