* fix: coerce null endpoint_url when delivering task result to a session
* fix: also coerce null model so the session insert satisfies NOT NULL
* test: cover task session delivery on an empty database
* feat(web-fetch): add web_fetch tool to read a specific URL's content
* test(web-fetch): add SSRF coverage and fail closed on empty DNS resolution
Add explicit SSRF regression tests for the web_fetch path covering
loopback, private LAN ranges, link-local/metadata, IPv6 private/local,
redirect-into-private, and unsupported schemes. Harden _public_http_url
to fail closed when a hostname resolves to no addresses.
