Add BambuPrinter::get_version, the first real request/response round trip

Documents request()'s real constraint on BambuMqttSession directly (not
all Bambu commands map 1:1 to a reply, and even where one exists its
shape can differ by protocol version - e.g. pushing.pushall triggers an
ongoing print.push_status stream rather than a direct reply, and
print.calibration data lives inside push_status on BambuMQTT v1 but gets
its own report on v2 - so request() is only correct for genuinely
one-shot, sequence_id-correlated commands like info.get_version).

get_version itself is Bambu-only (no GenericPrinter equivalent, matching
test_tls_handshake/fetch_certificate/ensure_trusted's existing pattern):
builds the request via continuum-schema-instance, sends it through
BambuMqttSession::request, and parses the reply into the existing
hand-written GetVersionResponse - reusing the struct already validated
against real captured Bambu JSON in bambu_commands_demo.rs, rather than
introducing a second response representation.

Verified structurally (no real broker reachable from here - confirmed
this sandbox has no route into the printer's LAN at all, not just a
blocked port): connect fails gracefully against an unreachable host, and
get_version correctly reports "not connected" rather than panicking when
called without a live session.

src/printer/bambu.rs

@@ -343,6 +343,33 @@ impl BambuPrinter {
             Err(err) => Err(err),
         }
     }
+
+    /// Bambu-only - no `GenericPrinter` equivalent, since every vendor's
+    /// way of reporting firmware info (if it has one at all) is shaped
+    /// completely differently. Same "vendor-only extension" pattern as
+    /// `test_tls_handshake`/`fetch_certificate`/`ensure_trusted` above:
+    /// reached by matching `PrinterHandle::Bambu` back out of the enum,
+    /// not through the shared trait.
+    ///
+    /// The first real use of `BambuMqttSession::request` -
+    /// `info.get_version` is a genuine one-shot, `sequence_id`-correlated
+    /// reply (unlike `pushing.pushall`, see `request`'s doc comment), so
+    /// this is exactly the case it's for.
+    pub async fn get_version(&self) -> Result<super::GetVersionResponse, PrinterError> {
+        let (session, version) =
+            self.connection.as_ref().ok_or_else(|| PrinterError(format!("{} is not connected", self.base.name)))?;
+
+        let body = continuum_schema_instance::build_instance_from_file(&get_schema_for_command(
+            *version,
+            "info".into(),
+            "get_version".into(),
+        ))
+        .map_err(|err| PrinterError(format!("failed to build get_version request: {err}")))?;
+
+        let response = session.request("info", body).await?;
+        serde_json::from_value(response)
+            .map_err(|err| PrinterError(format!("failed to parse get_version response: {err}")))
+    }
 }
 
 impl GenericPrinter for BambuPrinter {