fix: address codex review round 2

- Preserve abort errors while parsing search responses: when the caller's
  AbortSignal fires after headers but during response.json() (both the success
  and HTTP-error body parses), surface WEB_ABORTED instead of wrapping it as
  WEB_PROVIDER_ERROR, so agent cancel/dispose is not misreported as a provider
  failure. Applied to both the Exa and Perplexity providers.
- Report a malformed baseURL as misconfigured in status() (URL.canParse), so
  selection diagnostics and execution agree (configured-unavailable up front
  rather than a late WEB_PROVIDER_ERROR). WebProviderStatus already had the
  reason.

packages/web/web-search-perplexity/README.md

@@ -9,7 +9,7 @@ This is an **implementation** package: it registers a provider into `ctx.web`, i
 | Key | Default | Meaning |
 |---|---|---|
 | `apiKey` | `$PERPLEXITY_API_KEY` | Perplexity API key. Empty/absent → provider `status()` reports `missing-credential`. |
-| `baseURL` | `https://api.perplexity.ai` | Endpoint base; `/chat/completions` is appended. |
+| `baseURL` | `https://api.perplexity.ai` | Endpoint base; `/chat/completions` is appended. An unparseable value makes `status()` report `misconfigured`. |
 | `model` | `sonar` | Search model name. |
 
 ```yaml