Currently the web_search tool resolves a tenant-configured provider
chain (e.g. tavily → exa → brave) and stops at the first success.
That's the right default for cost/latency, but it makes cross-engine
corroboration impossible at the agent level: the caller has no way to
ask "search the same query on Exa specifically" once Tavily has
already returned a hit.
Add an optional `provider` argument. When set, the chain is narrowed
to the named provider (case-insensitive); other providers are not
queried. The first-success-wins fallback is preserved when the arg
is omitted.
Use case (concrete): a research agent verifying source freshness
calls web_search twice for the same query — once with provider="tavily"
and once with provider="exa" — and compares the URLs returned. URLs
that appear in both engines are high-confidence original primary
sources; URLs only in one signal a republish or low-circulation outlet
worth verifying further.
Other changes:
- Cache key now includes the requested provider so per-engine
results don't collide. Without this, the second call would just
replay the first engine's cached result, defeating the purpose.
- Unknown provider returns a clear error listing what IS configured
for the tenant — better DX than a silent no-op.
Tests: 5 new cases covering narrowing, case-insensitivity, unknown
provider, default first-success behaviour preservation, and cache
isolation across providers.
