docs: missing_docs sweep — 0 warnings + deny gate + publish-prep decisions (HY-02, HY-04, HY-11)

- document every public-API item across 18 files (openapi_spec model,
  HttpAuthScheme/HttpServiceConfig, HttpClientBuildError + SharedHttpClient
  accessors, RetryAfterMiddleware, GatewayDispatch, gateway error
  mapping, CallRequest/SchemaQuery/SubscribeStream, HttpAdapter +
  ALPNs + builders, decoy/healthz/state, WsSessions/WsPumps,
  from_openapi/from_jsonschema/from_mcp/from_wss/to_mcp, lib.rs module
  docs)
- enforcement: #![deny(missing_docs)] at crate root — stronger than CI
  rustdocflags (every build incl. cfg(test), where rustdoc misses the
  test-support module docs)
- HY-10 (opportunistic): all 8 docs.rs/alkhttp placeholder ADR links +
  the one relative ../docs link converted to plain text; the 10
  pre-existing private/redundant intra-doc-link warnings fixed —
  RUSTDOCFLAGS="-D warnings" cargo doc is fully clean
- HY-11 decision: docs/ + tasks/ excluded from the published package
  (contributor-facing design/process material; ADR references degrade
  to plain text uniformly). cargo publish --dry-run: 38 files, ~889 KiB,
  zero docs/ or tasks/ entries
- HY-04 decision: keep + document — frame_channel0_chunk's unwrap is
  on serializing the acyclic EventEnvelope (unreachable failure);
  # Panics on it and the adjacent WsClient senders state the contract

Verified: cargo test (299 + 5 TLS), --all-features (370 + suites),
--no-default-features (299), clippy --all-targets -D warnings
(default + all-features), fmt --check, cargo doc -D warnings clean,
cargo publish --dry-run --allow-dirty clean.

Tasks: review-001-missing-docs-sweep (final pending task; 42/42)

src/adapters/from_mcp/mod.rs

@@ -50,6 +50,9 @@ const MCP_CAPABILITY_KEY: &str = "mcp";
 /// timed out. Payload is the array-of-content-blocks shape; retryable.
 const MCP_TRANSPORT_ERROR: &str = "MCP_TRANSPORT_ERROR";
 
+/// The `from_mcp` adapter (mcp feature): imports a remote MCP server's
+/// tools as call-protocol operations under one namespace, with the
+/// auth token injected per-call from `Capabilities` (ADR-014).
 pub struct FromMCP {
     endpoint: String,
     auth_token: Option<Secret<String>>,
@@ -57,6 +60,8 @@ pub struct FromMCP {
 }
 
 impl FromMCP {
+    /// Assemble the adapter for a streamable-HTTP MCP `endpoint`,
+    /// registering its tools under `namespace`.
     pub fn new(endpoint: impl Into<String>, namespace: impl Into<String>) -> Self {
         Self {
             endpoint: endpoint.into(),
@@ -65,19 +70,24 @@ impl FromMCP {
         }
     }
 
+    /// Set the auth token sent as the MCP endpoint's `Authorization:
+    /// Bearer` header (wrapped in `Secret` so it never logs).
     pub fn with_auth_token(mut self, token: impl Into<String>) -> Self {
         self.auth_token = Some(Secret::new(token.into()));
         self
     }
 
+    /// The configured MCP endpoint URL.
     pub fn endpoint(&self) -> &str {
         &self.endpoint
     }
 
+    /// The namespace imported operations register under.
     pub fn namespace(&self) -> &str {
         &self.namespace
     }
 
+    /// The configured token, for introspection (never logged).
     pub fn auth_token(&self) -> Option<&Secret<String>> {
         self.auth_token.as_ref()
     }