[serve] Read the multiplex marker statically so probing cannot initialize handles

[johntaylor-cell]

Follow on to PR #64045

Why

_callable_uses_multiplexing() probed instance attributes with getattr(), which on a
DeploymentHandle triggers __getattr__ → options() → eager Router initialization.
Under direct ingress this scan runs at startup on every ingress replica, so each one
holding a handle built a Router that would otherwise stay lazy — each adding a
controller long-poll subscription and metric-report stream. Cost scales with replicas.

What

Both probes in the scan are now side-effect free:

• The marker is read with inspect.getattr_static(), which never invokes __getattr__.
• The instance __dict__ is read with object.__getattribute__() rather than
  getattr(obj, "__dict__", {}), which falls back to __getattr__ on a __slots__
  class. getattr_static is not usable here — for __dict__ it returns the type's
  getset_descriptor rather than the instance mapping.

Detection is unchanged. Adds a regression test asserting the scan never invokes
__getattr__, covering both a handle-holding callable and a __slots__ callable; the
existing test only covers false positives, so it cannot catch either.

Test

4096-replica controller benchmark, HAProxy on, vs the immediate parent commit (n=1 per
arm): handle metric report delay 7275ms → 827ms, deployment_state_update
0.242s → 0.151s, controller asyncio tasks 12182 → 10004.

[gemini-code-assist]

Code Review

This pull request replaces the use of getattr with inspect.getattr_static when checking for the multiplexed function marker in _callable_uses_multiplexing. This prevents unintended side effects, such as eagerly initializing DeploymentHandle Routers during probing. A unit test has also been added to verify this behavior. The reviewer suggests a further improvement: applying inspect.getattr_static to retrieve __dict__ later in the same function to ensure the entire probing process is completely side-effect-free.

[abrarsheikh]

why does getattr cause options to be called?

[johntaylor-cell]

why does getattr cause options to be called?

utils.py:76        getattr(obj, MULTIPLEXED_FUNCTION_MARKER_ATTR, False)
                   └─ obj is a DeploymentHandle (the replica's self.hello_world)
                   └─ normal lookup FAILS: not in instance __dict__, not in type MRO
                        ↓  Python falls back to __getattr__
handle.py:304      DeploymentHandle.__getattr__(self, name)
handle.py:305          return self.options(method_name=name)
                        ↓
handle.py:1141     options(...) → return self._options(...)
                        ↓
handle.py:223-224  _options():  if not self.is_initialized:
                                    self._init() <-------------------

Accept added delay and close PR? Or merge PR and fix benchmark to send requests to the replicas?