Add v6 to v7 AI migration guide (#6387)

* docs(ai): add v6 to v7 AI migration guide

Machine-oriented migration guide for AI agents migrating Android Chat
SDK from v6 to v7. Covers dependencies, plugin system removal, theming
redesign, package relocations, component renames, and verification.

Tested by having an AI agent migrate the v6 compose sample app to v7
with zero prior SDK knowledge — builds successfully.

* fix(ai): address PR review comments

- Fix duplicated detection regex (MessageContentFactory → ChatComponentFactory)
- Use portable perl -i -pe for bulk drawable rename (works on macOS + Linux)
- Fix MentionSuggestionItem/List JSON mappings (null → actual v7 names)
- Complete drawableRenames JSON with all mappings from §5.8

* docs(ai): add Windows platform note

* fix(ai): address reviewer feedback from @VelikovPetar

- Fix ChatClientConfig mapping table: correct v6 sources for each param,
  fix userPresence default (true not false), clarify backgroundSyncEnabled
  is removed (not mapped to isAutomaticSyncOnReconnectEnabled)
- Replace hardcoded version numbers with "check Maven Central"
- Add removed lambda parameters for ChannelList, MessageList,
  ChannelScreen, ChannelListHeader, ChannelHeader (PR #6365)
- Document ChatComponentFactory as the replacement for all slot params
  with link to migration docs
- Add cross-reference between §3.4 and §7.2 for changed defaults

* fix(ai): correct isAutomaticSyncOnReconnectEnabled v6 source

Author: aleksandar-apostolov