* fix: custom web search — WEB_URL_TEMPLATE not recognized, timeout too short, silent native fallback
1. custom.ts: Add WEB_URL_TEMPLATE to isConfigured() so the custom provider
is recognized when configured via URL template alone.
2. custom.ts: Bump DEFAULT_TIMEOUT_SECONDS from 15s to 120s.
Self-hosted search APIs (SearXNG, internal) commonly need 30-90s.
3. WebSearchTool.ts: When an explicit adapter is selected via
WEB_SEARCH_PROVIDER=custom, do not silently fall through to the
native Anthropic path on adapter errors or 0-hit results.
- 0 hits: return directly (no fallback)
- Error: throw the real error (no fallback)
- Auto mode: existing fallback behavior preserved
* fix: tighten auto-mode adapter fallback — only swallow transient errors
Address review feedback: in auto mode, only fall through to native on
transient errors (network failure, timeout, HTTP 5xx). Config and
guardrail errors (SSRF, HTTPS, bad URL, header allowlist, etc.) now
surface properly instead of being silently swallowed.
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Co-authored-by: FluxLuFFy <fluxluffy@users.noreply.github.com>
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