fix: keep the code of version 6 working and document what still breaks

[sergioteula]

The clients gained arguments and errors gained types since version 6, and
some of that broke code that had no reason to break. Three changes give it
back without giving up what the new behaviour is for:

• availability sat between item_page and condition in search_items, so a
  caller not using keywords bound every following value to the wrong
  argument, silently. It moves to the end and becomes keyword only, which
  also keeps it from displacing anything in the future.
• A request that fails to authenticate or is denied raised RequestError
  before it got its own type, so AuthenticationError and AccessDeniedError
  become subclasses of it.
• An invalid value raised the ValidationError of pydantic or a plain
  ValueError, both ValueError subclasses, so InvalidArgumentError becomes
  one too. An unsupported version now raises it as well, instead of the
  plain ValueError it kept for no reason.

The signatures of both clients and the hierarchy of the errors are pinned
by tests, as they are the promise this commit is making.

What is left breaking is what cannot be given back without restoring a bug,
and is now covered by a migration guide: the errors mapped from the
response, the values the async client no longer sends unvalidated, the
items not found, the timeout and the retries of the requests, the ASIN of a
malformed URL and the configuration each client keeps for itself.

Co-Authored-By: Claude Opus 5 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_017bWGetbCpBr2upxna7R5QV