refactor(types): canonicalize shared provider settings identifiers (#1109)

* refactor(types): centralize provider model definitions

* fix(types): preserve provider settings compatibility

* refactor(types): derive model keys from provider definitions

* docs(types): clarify model key compatibility

* fix(types): preserve legacy model id keys

* fix(types): preserve model id key compatibility

* fix(core): adopt active provider type guard

* fix(types): preserve provider guard compatibility

* refactor(types): retain concise provider guard name

* test(types): clarify provider model ID selection

* test(types): simplify model ID key contract

* refactor(types): make provider model accessors explicit

* refactor(types): decompose provider settings

* refactor(types): colocate provider setting keys

* test(webview): use canonical provider identifiers

* docs(types): explain model provider exclusions