feat: add SEP-2575 meta helpers (#942)

Expose typed accessors for the per-request protocol version, client info,
client capabilities, and log level entries carried in _meta without making
those fields required for older peers.

Fixes #869

crates/rmcp/tests/test_meta_helpers.rs

@@ -0,0 +1,73 @@
+#![allow(deprecated)]
+
+use rmcp::model::{ClientCapabilities, Implementation, LoggingLevel, Meta, ProtocolVersion};
+use serde_json::json;
+
+const META_KEY_PROTOCOL_VERSION: &str = "io.modelcontextprotocol/protocolVersion";
+const META_KEY_CLIENT_INFO: &str = "io.modelcontextprotocol/clientInfo";
+const META_KEY_CLIENT_CAPABILITIES: &str = "io.modelcontextprotocol/clientCapabilities";
+const META_KEY_LOG_LEVEL: &str = "io.modelcontextprotocol/logLevel";
+
+#[test]
+fn meta_setters_store_sep_2575_values() {
+    let mut meta = Meta::new();
+    meta.set_protocol_version(ProtocolVersion::V_2026_07_28);
+    meta.set_client_info(Implementation::new("test-client", "1.0.0"));
+    meta.set_client_capabilities(ClientCapabilities::default());
+    meta.set_log_level(LoggingLevel::Warning);
+
+    assert_eq!(
+        meta.get(META_KEY_PROTOCOL_VERSION),
+        Some(&json!("2026-07-28"))
+    );
+    assert_eq!(
+        meta.get(META_KEY_CLIENT_INFO),
+        Some(&json!({ "name": "test-client", "version": "1.0.0" }))
+    );
+    assert_eq!(meta.get(META_KEY_CLIENT_CAPABILITIES), Some(&json!({})));
+    assert_eq!(meta.get(META_KEY_LOG_LEVEL), Some(&json!("warning")));
+}
+
+#[test]
+fn meta_accessors_decode_wire_values() {
+    let meta: Meta = serde_json::from_value(json!({
+        "progressToken": "progress-1",
+        "io.modelcontextprotocol/protocolVersion": "2026-07-28",
+        "io.modelcontextprotocol/clientInfo": {
+            "name": "wire-client",
+            "version": "9.0.0"
+        },
+        "io.modelcontextprotocol/clientCapabilities": {
+            "sampling": {}
+        },
+        "io.modelcontextprotocol/logLevel": "error"
+    }))
+    .unwrap();
+
+    assert_eq!(meta.protocol_version(), Some(ProtocolVersion::V_2026_07_28));
+    assert_eq!(
+        meta.client_info(),
+        Some(Implementation::new("wire-client", "9.0.0"))
+    );
+    assert!(
+        meta.client_capabilities()
+            .is_some_and(|capabilities| capabilities.sampling.is_some())
+    );
+    assert_eq!(meta.log_level(), Some(LoggingLevel::Error));
+}
+
+#[test]
+fn meta_accessors_ignore_missing_or_malformed_values() {
+    let meta: Meta = serde_json::from_value(json!({
+        "io.modelcontextprotocol/protocolVersion": 20260728,
+        "io.modelcontextprotocol/clientInfo": "not an implementation",
+        "io.modelcontextprotocol/clientCapabilities": "not capabilities",
+        "io.modelcontextprotocol/logLevel": "loud"
+    }))
+    .unwrap();
+
+    assert_eq!(meta.protocol_version(), None);
+    assert_eq!(meta.client_info(), None);
+    assert_eq!(meta.client_capabilities(), None);
+    assert_eq!(meta.log_level(), None);
+}