fix(review): harden the E2E-gap fixes after security/regression review

Addresses findings from an independent review of the gap-closing commits:

- R1 (regression): add #[serde(default)] to HttpDispatchPayload.method so
  async-HTTP outbox rows enqueued before the field existed still decode
  after upgrade instead of dead-lettering on "missing field `method`".
  Adds a regression test for the missing-key path.
- method case: uppercase ctx.request.method at the orchestrator boundary
  so it honors its documented "uppercased" contract for extension verbs.
  Route matching is already case-insensitive, so matching is unaffected.
- CLI hardening: percent-encode free-form path segments (app slug, topic
  name, ids, secret name) in the reqwest client so a value containing
  `/ ? #` can't break out of its URL segment. Adds a `seg()` helper +
  unit test and applies it uniformly across all path-interpolating
  methods (new and pre-existing).
- S1 (docs): correct the users::email_available enumeration framing — it
  adds no new vector vs. create's uniqueness error, but is cheaper and
  unthrottled; there is no built-in throttle/CAPTCHA primitive, so the
  honest mitigation is a kv-based counter. Updated SDK doc-comments,
  trait docs, and stdlib-reference.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/shared/src/users.rs

@@ -207,6 +207,13 @@ pub trait UsersService: Send + Sync {
     /// this leaks only a single boolean — no id, profile, or timing
     /// distinction beyond "exists / doesn't" — so a public registration
     /// script can pre-check before calling `create`.
+    ///
+    /// Enumeration note: that bit is the same one a register form already
+    /// leaks ("email taken"), but this probe is cheaper (no password hash),
+    /// has no side effect (a `create` miss writes a junk row), and is not
+    /// rate limited (there is no built-in throttle/CAPTCHA primitive). A
+    /// script exposing it on an anonymous route inherits account-enumeration
+    /// risk and must add its own `kv`-based throttle if that matters.
     async fn email_available(&self, cx: &SdkCallCx, email: &str) -> Result<bool, UsersError>;
 
     async fn update(