test: expand provider and tool error-path coverage

[laynepenney]

What

• add focused unit tests for provider error-response mapping in OpenAI and Anthropic providers
• add tool handler validation tests for read_file (offset/limit bounds, offset beyond file length)
• add tool handler input-shape tests for write_file (missing/invalid content)

Why

This closes concrete Phase 3 readiness gaps around edge-case and failure-path behavior, especially around unknown API error payloads and malformed tool inputs.

Validation

• cargo test --locked