Merge bitcoin/bitcoin#34437: rpc: uptime should begin on application start

e67a676df9 fix: uptime RPC returns 0 on first call (Lőrinc)

Pull request description:

  ### Problem
  #34328 switched uptime to use monotonic time, but `g_startup_time` was a function-local static in `GetUptime()`, meaning it was initialized on first call rather than at program start.
  This caused the first uptime RPC to always return 0.

  ### Fix
  Move `g_startup_time` to namespace scope so it initializes at program start, ensuring the first `uptime()` call returns actual elapsed time.

  ### Reproducer

  Revert the fix and run the test or alternatively:

  ```bash
  cmake -B build && cmake --build build --target bitcoind bitcoin-cli -j$(nproc)
  ./build/bin/bitcoind -regtest -daemon
  sleep 10
  ./build/bin/bitcoin-cli -regtest uptime
  ./build/bin/bitcoin-cli -regtest stop
  ```

  <details>
  <summary>Before (uptime is initialized on first call)</summary>

  ```bash
  Bitcoin Core starting
  0
  Bitcoin Core stopping
  ```

  </details>

  <details>
  <summary>After (first uptime call is in-line with sleep)</summary>

  ```bash
  Bitcoin Core starting
  10
  Bitcoin Core stopping
  ```
  </details>

  ----

  Fixes #34423, added reporter as coauthor.

ACKs for top commit:
  maflcko:
    lgtm ACK e67a676df9
  optout21:
    crACK e67a676df9
  carloantinarella:
    Tested ACK e67a676df9
  achow101:
    ACK e67a676df9
  musaHaruna:
    Tested ACK [e67a676](e67a676df9)

Tree-SHA512: b156f7ed15c3fbb50e8a15f6c9a0d4e2ffb956d0c6ef949e0f8a661a564a20c0d3ed2149fae75ce7e2baa9326788d5037e726e7a7ac2c6ef4e70e4862cd5763f

src/common/system.cpp

@@ -127,8 +127,8 @@ std::optional<size_t> GetTotalRAM()
     return std::nullopt;
 }
 
-SteadyClock::duration GetUptime()
-{
-    static const auto g_startup_time{SteadyClock::now()};
-    return SteadyClock::now() - g_startup_time;
-}
+namespace {
+    const auto g_startup_time{SteadyClock::now()};
+} // namespace
+
+SteadyClock::duration GetUptime() { return SteadyClock::now() - g_startup_time; }

test/functional/rpc_uptime.py

@@ -26,8 +26,11 @@ class UptimeTest(BitcoinTestFramework):
         assert_raises_rpc_error(-8, "Mocktime must be in the range [0, 9223372036], not -1.", self.nodes[0].setmocktime, -1)
 
     def _test_uptime(self):
-        wait_time = 20_000
+        time.sleep(1) # Do some work before checking uptime
         uptime_before = self.nodes[0].uptime()
+        assert uptime_before > 0, "uptime should begin at app start"
+
+        wait_time = 20_000
         self.nodes[0].setmocktime(int(time.time()) + wait_time)
         uptime_after = self.nodes[0].uptime()
         self.nodes[0].setmocktime(0)