fix(api): bound the stream drain so a full config dump can't starve the watchdog (#11164)

writeStream() drained the whole queue in one call - "send every packet we can".
A client asking for the full config gets the node database, then the file
manifest, then the packet backlog and the position replay, and none of that
returns to loop(). On a full node database (120 entries) the dump runs past
eight seconds, so on RP2350, where rp2040Loop() arms an 8s hardware watchdog
and is the only thing that calls watchdog_update(), the board resets in the
middle of the manifest. Reproducible on every connection; with a small node
database the dump finished under the timeout and nothing looked wrong.

Measured on a pico2_w5500_e22: last loop iteration at millis=27242, ServerAPI
kept logging until uptime 35s, reset at 35.2s = 27.242 + 8.0.

Take a slice instead. The PhoneAPI state machine is resumable, so writeStream()
stops after STREAM_WRITE_BUDGET_MSEC and reports whether anything is left;
runOncePart() then asks to be re-run immediately rather than sleeping out
readStream's idle delay, so the dump keeps its throughput while loop() gets to
feed the watchdog between slices. Backpressure on a retained frame still
returns the normal delay - re-running at once would just spin on a full
transport.

Verified on hardware: 10/10 full config dumps against a node with 120 entries,
no resets, dump still completes in ~8s.

src/mesh/StreamAPI.cpp

@@ -13,7 +13,9 @@
 int32_t StreamAPI::runOncePart()
 {
     auto result = readStream();
-    writeStream();
+    // More to send: come straight back instead of sleeping out readStream's idle delay.
+    if (writeStream())
+        result = 0;
     checkConnectionTimeout();
     return result;
 }
@@ -22,7 +24,8 @@ int32_t StreamAPI::runOncePart()
 int32_t StreamAPI::runOncePart(char *buf, uint16_t bufLen)
 {
     auto result = readStream(buf, bufLen);
-    writeStream();
+    if (writeStream())
+        result = 0;
     checkConnectionTimeout();
     return result;
 }
@@ -44,25 +47,29 @@ int32_t StreamAPI::readStream(const char *buf, uint16_t bufLen)
     }
 }
 
-/**
- * call getFromRadio() and deliver encapsulated packets to the Stream
- */
-void StreamAPI::writeStream()
+/// Emit a slice of pending output. True means "more to send, come straight back"; false covers
+/// both a drained queue and backpressure, where retrying at once would only spin.
+bool StreamAPI::writeStream()
 {
-    if (canWrite) {
-        // A transport that retained a short frame must complete it before
-        // getFromRadio() advances the PhoneAPI state to the next packet.
-        if (!finishPendingFrame())
-            return;
+    if (!canWrite)
+        return false;
 
-        uint32_t len;
-        do {
-            // Send every packet we can
-            len = getFromRadio(txBuf + HEADER_LEN);
-            if (len != 0 && !emitTxBuffer(len))
-                break;
-        } while (len);
-    }
+    // A retained short frame must complete before getFromRadio() advances the PhoneAPI state.
+    if (!finishPendingFrame())
+        return false;
+
+    // Draining a full dump in one call never returns to loop(), so the 8s hardware watchdog
+    // fires mid-dump. PhoneAPI is resumable, so stop at the budget and continue next dispatch.
+    uint32_t len;
+    uint32_t started = millis();
+    do {
+        // Send every packet we can, up to this slice's budget
+        len = getFromRadio(txBuf + HEADER_LEN);
+        if (len != 0 && !emitTxBuffer(len))
+            return false;
+    } while (len && Throttle::isWithinTimespanMs(started, STREAM_WRITE_BUDGET_MSEC));
+
+    return len != 0;
 }
 
 /// Parse supplied bytes through the framed ToRadio receive state machine.

src/mesh/StreamAPI.h

@@ -9,6 +9,10 @@
 // A To/FromRadio packet + our 32 bit header
 #define MAX_STREAM_BUF_SIZE (MAX_TO_FROM_RADIO_SIZE + sizeof(uint32_t))
 
+// Cap on one writeStream() slice: an uncapped dump never reaches loop(), so a board with a
+// hardware watchdog (RP2350 arms 8s) resets mid-dump.
+#define STREAM_WRITE_BUDGET_MSEC 100
+
 /**
  * A version of our 'phone' API that talks over a Stream.  So therefore well suited to use with serial links
  * or TCP connections.
@@ -64,10 +68,9 @@ class StreamAPI : public PhoneAPI
     int32_t readStream(const char *buf, uint16_t bufLen);
     int32_t handleRecStream(const char *buf, uint16_t bufLen);
 
-    /**
-     * call getFromRadio() and deliver encapsulated packets to the Stream
-     */
-    void writeStream();
+    /// Emit a slice of pending output. True asks the caller to come straight back; false covers
+    /// both a drained queue and backpressure, so it does not mean the queue is empty.
+    bool writeStream();
 
   protected:
     /**