feat!: add MRTR behavior support (SEP-2322) (#929)

* feat!: add MRTR behavior support

* feat: harden SEP-2322 MRTR support

* ci: diff public API only on features common to base and head

cargo public-api builds both revisions with the same feature set, so a
feature introduced (or removed) by a PR broke the build of the other
revision. Restrict the all-features diff to features present in both the
base and head revisions.

crates/rmcp/Cargo.toml

@@ -22,6 +22,7 @@ features = [
   "client-side-sse",
   "elicitation",
   "macros",
+  "request-state",
   "reqwest",
   "reqwest-native-tls",
   "reqwest-tls-no-provider",
@@ -64,6 +65,10 @@ schemars = { version = "1.0", optional = true, features = ["chrono04"] }
 # for image encoding
 base64 = { version = "0.22", optional = true }
 
+# for SEP-2322 requestState integrity sealing (opt-in via the `request-state` feature)
+hmac = { version = "0.12", optional = true }
+sha2 = { version = "0.10", optional = true }
+
 # for HTTP client
 reqwest = { version = "0.13.2", default-features = false, features = [
   "json",
@@ -120,6 +125,9 @@ server = ["transport-async-rw", "schemars", "dep:pastey"]
 macros = ["dep:rmcp-macros", "dep:pastey"]
 elicitation = ["dep:url"]
 
+# SEP-2322 requestState integrity helper (HMAC-SHA256 seal/open codec)
+request-state = ["dep:hmac", "dep:sha2", "base64"]
+
 # reqwest http client
 __reqwest = ["dep:reqwest"]
 
@@ -315,6 +323,11 @@ name = "test_trace_context"
 required-features = ["server", "client"]
 path = "tests/test_trace_context.rs"
 
+[[test]]
+name = "test_mrtr_behavior"
+required-features = ["server", "client"]
+path = "tests/test_mrtr_behavior.rs"
+
 [[test]]
 name = "test_prompt_macros"
 required-features = ["server", "client"]