refactor(webview): use canonical provider identifiers (#1023)

* refactor(webview): use canonical provider identifiers

* test(webview): remove brittle identifier mutation tests

* fix(webview): handle remaining model providers explicitly

* test(webview): avoid unreachable coverage branch

* refactor(webview): use canonical anthropic identifier

* test(webview): cover Anthropic Opus 1M tier

* test(webview): use canonical Anthropic identifier

* fix(webview): address canonical provider review

* test(webview): use canonical provider identifiers

* test(webview): cover Kimi Code model selection

Author: WebMad