http_service::run mapped `Method::from_bytes` failure on a user-supplied
HTTP method to HttpError::Backend with format!("invalid method: {}", …).
Same for HeaderName/HeaderValue validation in build_headers. But that's
user input, not a backend problem — misclassifying as Backend corrupts
retry policies (operators may retry "backend" but not "invalid input").
Add a new HttpError::InvalidArgs(String) variant. Reroute the three
validation paths in http_service (method, header name, header value) to
emit InvalidArgs instead of Backend.
The executor-side `map_http_err` uses Display, so the new variant
surfaces to scripts as "http: invalid method: …" — same format, more
structured behind it.
AUDIT.md anchor: F-Q-008.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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