fix(serial): don't sleep forever with pending PhoneAPI output on UART consoles (#11500)

* fix(serial): don't sleep forever with pending PhoneAPI output on UART consoles

Since #11164 bounded the stream drain, a config dump can end a dispatch
with output still queued. On UART-console ESP32 boards runOnce() then
returns INT32_MAX with no RX pending, and neither rxInt() nor
onNowHasData() fires for the remaining output, so the download wedges
mid nodeinfo stream until the client happens to send a byte.

Add StreamAPI::hasPendingOutput() (transport-retained frame or queued
PhoneAPI data) and have SerialConsole::runOnce() short-poll (<=25ms)
while it holds instead of sleeping INT32_MAX. The #11164 write budget
is unchanged; idle sleep behavior with a drained queue is unchanged.
The retained-frame probe also covers the ESP32-S2 USB-CDC branch,
which takes the same INT32_MAX path.

* test(serial): restore scratch NodeDB via tearDown, trim comments to house style

A failed TEST_ASSERT longjmps out of a Unity test without running
destructors, so RAII cannot restore the swapped nodeDB pointer; install
the scratch NodeDB explicitly and restore/delete it in tearDown(),
which runs after every test outcome. Also shorten the new comments to
the two-line house limit.

src/mesh/StreamAPI.h

@@ -57,6 +57,10 @@ class StreamAPI : public PhoneAPI
     virtual int32_t runOncePart();
     virtual int32_t runOncePart(char *buf, uint16_t bufLen);
 
+    /// True while undelivered output remains (retained frame or queued PhoneAPI data); callers
+    /// woken only by RX activity must keep polling while set, as drains stop mid-dump (#11164).
+    bool hasPendingOutput();
+
     /// Check the current underlying physical link to see if the client is currently connected
     virtual bool checkIsConnected() override = 0;
 
@@ -104,6 +108,8 @@ class StreamAPI : public PhoneAPI
 
     /// Complete retained transport output before dequeuing another PhoneAPI packet.
     virtual bool finishPendingFrame() { return true; }
+    /// Return whether the transport retains an incomplete frame awaiting TX space.
+    virtual bool hasRetainedFrame() { return false; }
     /// Return whether the dedicated log buffer is available for encoding.
     virtual bool canEncodeLogRecord() { return true; }
     /// Frame and write a payload, optionally using best-effort admission.