agent: Improve error handling and retry for zed-provided models (#33565)

* Updates to `zed_llm_client-0.8.5` which adds support for `retry_after`
when anthropic provides it.

* Distinguishes upstream provider errors and rate limits from errors
that originate from zed's servers

* Moves `LanguageModelCompletionError::BadInputJson` to
`LanguageModelCompletionEvent::ToolUseJsonParseError`. While arguably
this is an error case, the logic in thread is cleaner with this move.
There is also precedent for inclusion of errors in the event type -
`CompletionRequestStatus::Failed` is how cloud errors arrive.

* Updates `PROVIDER_ID` / `PROVIDER_NAME` constants to use proper types
instead of `&str`, since they can be constructed in a const fashion.

* Removes use of `CLIENT_SUPPORTS_EXA_WEB_SEARCH_PROVIDER_HEADER_NAME`
as the server no longer reads this header and just defaults to that
behavior.

Release notes for this is covered by #33275

Release Notes:

- N/A

---------

Co-authored-by: Richard Feldman <oss@rtfeldman.com>
Co-authored-by: Richard <richard@zed.dev>

crates/assistant_context/src/assistant_context.rs

@@ -2140,7 +2140,8 @@ impl AssistantContext {
                                         );
                                     }
                                     LanguageModelCompletionEvent::ToolUse(_) |
-                                    LanguageModelCompletionEvent::UsageUpdate(_)  => {}
+                                    LanguageModelCompletionEvent::ToolUseJsonParseError { .. } |
+                                    LanguageModelCompletionEvent::UsageUpdate(_) => {}
                                 }
                             });