docs(memory): use public preload_memory tool API in examples

[garyzava]

Summary

The Memory docs and the express-mode weather agent notebook show the preload
tool as adk.tools.preload_memory_tool.PreloadMemoryTool(). Copy-pasting that
throws AttributeError: module 'google.adk.tools' has no attribute 'preload_memory_tool'. This PR changes the Python examples to the public API,
from google.adk.tools import preload_memory, which is also how load_memory
is shown on the same page.

Fixes #1771

What changed

• docs/sessions/memory.md - both Python tabs now import preload_memory and
  pass the preload_memory instance in tools=[...].
• examples/python/notebooks/express-mode-weather-agent.ipynb - same fix in the
  import cell and the agent definition.

Go, Java, Kotlin and TypeScript tabs are left as-is.

Why

google.adk.tools exports the tool through a lazy __getattr__ as the instance
preload_memory (from .preload_memory_tool import preload_memory_tool as preload_memory). The preload_memory_tool submodule isn't an attribute of the
package, so the documented adk.tools.preload_memory_tool.PreloadMemoryTool()
fails. Using preload_memory directly matches the existing load_memory
example (from google.adk.tools import load_memory -> tools=[load_memory]).

How I tested

Checked against google-adk==2.1.0:

# new form (what this PR documents) works:
>>> from google.adk.tools import preload_memory
>>> type(preload_memory).__name__, preload_memory.name
('PreloadMemoryTool', 'preload_memory')

# old documented form raises the error from #1771:
>>> from google import adk
>>> adk.tools.preload_memory_tool.PreloadMemoryTool()
AttributeError: module 'google.adk.tools' has no attribute 'preload_memory_tool'.
                Did you mean: 'preload_memory'?

mkdocs build passes, and the rendered sessions/memory/ page now only shows
preload_memory.

Backward compatibility

Docs/examples only. from google.adk.tools.preload_memory_tool import PreloadMemoryTool still works; this just documents the supported public form.
No code changes.

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