Fix deadlock in Measurements.close() and add test coverage

[jaisinha77777]

Summary

While adding test coverage for concordia/utils/measurements.py (part of #205, no test file previously existed), I found that Measurements.close() deadlocks:

def close(self) -> None:
  with self._channels_lock:
    for channel in self._channels:
      self.close_channel(channel)   # tries to acquire _channels_lock again
    self._channels.clear()

def close_channel(self, channel: str) -> None:
  with self._channels_lock:
    del self._channels[channel]

_channels_lock is a plain threading.Lock (not reentrant), so close() blocks forever the moment it calls close_channel() while already holding the lock. Reproducible any time there's at least one open channel:

m = Measurements()
m.publish_datum('a', 1)
m.close()  # hangs forever

The loop also mutates self._channels while iterating over it, a second independent bug (RuntimeError: dictionary changed size during iteration) masked by the deadlock.

Separately, close_channel()'s docstring documents no-op behavior for a channel that doesn't exist ("If the channel doesn't exist yet, it will be created"), but the implementation (del self._channels[channel]) raises KeyError in that case.

There don't appear to be any current callers of close()/close_channel() in the repo, which is presumably why this has gone unnoticed.

Fix

• close() now just clears the dict directly under the lock, without calling back into close_channel().
• close_channel() uses dict.pop(channel, None) so it's idempotent on a missing channel, matching its docstring.

Test plan

• Added concordia/utils/measurements_test.py, including a regression test that runs close() on a background thread and asserts it completes within a timeout (hangs/fails before this fix, passes after).
• Added concordia/utils/async_measurements_test.py covering ReactiveMeasurements (capture/subscribe/dispose semantics, plus confirming close() doesn't deadlock on the subclass either).
• python -m pytest concordia/utils/ -q -> all passing.
• python -m pyink --check on all changed files -> clean.