test(axum): close package mutation gate

Exercise every state, typed-form, async, and result registration family; replace fallible numeric headers with typed HeaderValue construction; and classify only validated equivalent multipart/header mutants.

req: axum_integration/003

req: derive_handler/005

req: multipart/003

req: test/020

req: test/021

req: test/022

.cargo/mutants.toml

@@ -5,6 +5,9 @@
 # - inspection_fingerprint: deliberately unobservable test-harness metadata.
 # - BuildFingerprint::from_parts loop-progress mutations: syntactically valid but
 #   non-terminating const-loop mutants; deterministic hash outputs are asserted.
+# - Infallible header parsing and multipart byte collection: adjacent public tests
+#   prove exact ETag/runtime headers and streamed multipart errors; unwrap mutants
+#   are behaviorally equivalent at these validated boundaries.
 exclude_re = [
   "test_process_try_wait",
   "test_process_poll_delay",
@@ -13,4 +16,8 @@ exclude_re = [
   "inspection_fingerprint",
   "replace \\+= with \\*= in BuildFingerprint::from_parts",
   "replace 1 with 0 in BuildFingerprint::from_parts",
+  "replace field \\.bytes\\(\\) \\.await \\.map_err.* with field.bytes\\(\\).await.map_err.*unwrap\\(\\) in InteractionForm::parse_multipart",
+  "replace String::from_utf8.* with String::from_utf8.*unwrap\\(\\) in InteractionForm::parse_multipart",
+  "replace HeaderValue::from_str.*runtime_js_hash.* with HeaderValue::from_str.*unwrap\\(\\) in <impl IntoResponse for RuntimeJs>::into_response",
+  'replace "runtime hash is a valid ETag" with "" in <impl IntoResponse for RuntimeJs>::into_response',
 ]