fix(http): F-Q-008 add HttpError::InvalidArgs for user-input validation failures

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>

crates/shared/src/http.rs

@@ -109,6 +109,13 @@ pub enum HttpError {
     #[error("{0}")]
     Network(String),
 
+    /// F-Q-008: user-supplied input failed validation (bad HTTP method
+    /// name, malformed header name/value). Distinct from `Backend`
+    /// because operators may safely retry a `Backend` error but should
+    /// never retry an `InvalidArgs` failure — the input itself is wrong.
+    #[error("{0}")]
+    InvalidArgs(String),
+
     /// Anything else the impl wants to surface (still safe to show a
     /// script).
     #[error("{0}")]