fix(client): align date filter inputs with api

[Shedna]

Description

The public gateway accepts fromDate and toDate as ISO date strings and also normalizes full ISO datetime strings on search and research, but the Python client surface still typed those parameters as date only.

This PR synchronizes the Python SDK by:

• updating search / async_search / research / async_research to accept date | str | None,
• documenting the accepted date-filter formats in client docstrings and the README,
• adding focused tests that cover ISO datetime string payloads for search and research.

Checklist

• I have installed prek on this project (for instance with the make install-dev command)
  before creating any commit, or I have run successfully the make lint command on my changes.
• I have run successfully the make typecheck test command on my changes.
• I have updated the README.md if my changes affected it.

[cjumel]

For the tracking purposes: the PR just updated the README whereas it was not relevant, so I fixed that and updated the AGENTS.md to try and prevent such updates