fix(web_fetch): report transport failure causes

[sheperdh]

Summary

• classify web_fetch request and response-body failures while preserving the existing stable error prefixes
• expose the deepest transport cause to the tool result and emit a bounded full source chain in structured logs
• add connection, timeout, and diagnostic-size regression coverage

Validation

• cargo fmt --all -- --check
• cargo test -p zeroclaw-tools web_fetch::tests
• cargo clippy -p zeroclaw-tools --all-targets --no-deps -- -D warnings -A clippy::useless-format

The unmodified workspace-wide clippy gate is currently blocked by an existing chunks_exact_to_as_chunks lint in zeroclaw-memory/src/vector.rs. The full zeroclaw-tools suite has four unrelated environment/concurrency failures.

[Kevin-K-W]

Reviewed the web_fetch transport-cause change. Prefix compatibility and fallback behavior look correct; three suggestions below (response-body regression coverage, extracting the diagnostic helpers, and aligning cause_chain redaction).