Adds a shared classifyProviderHttpError helper that maps the well-known
failure shapes (status 401/402/403 and bodies matching credits / quota
/ insufficient) into compact SearchProviderError messages so the
orchestrator advances to the next provider instead of bailing.
Wired into every HTTP-talking provider (codex, exa, gemini, anthropic,
brave, jina, kimi, perplexity, searxng, synthetic, tavily, zai) and the
two wrapped-error providers (kagi via KagiApiError, parallel via
ParallelApiError). The orchestrator now collects per-provider failures
and emits a joined summary ("exa: 403 forbidden; codex: credits
exhausted; ...") when the whole chain fails.
Test updates: web-search-{exa,tavily,kagi} message expectations now
assert the compact "<id>: <status> <reason>" form.
can1357
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