fix: strengthen fetch_url IP validation

[awhite0030]

Description

Brief description of what this PR does

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Changeset

• Added a changeset (pnpm changeset) describing this change for the changelog

Docs-only or internal chores need no changeset (or run pnpm changeset --empty to note that intentionally).

Testing

Automated Tests

• New features include passing tests in .spec.ts/tsx files
• All existing tests pass (pnpm test:all completes successfully)
• Tests cover both success and error scenarios

Manual Testing

• Tested with Ollama
• Tested with OpenRouter
• Tested with OpenAI-compatible API
• Tested MCP integration (if applicable)

Checklist

• If this was for an open issue, I was assigned to it
• Code follows project style guidelines
• Self-review completed
• Documentation updated (if needed)
• No breaking changes (or clearly documented)
• Appropriate logging added using structured logging (see CONTRIBUTING.md)

---

Root cause: The validation for fetch_url's loopback and internal network check was relying on strict string matching for exact values (e.g., 127.0.0.1, localhost) rather than correctly analyzing the normalized hostname as an IPv4/IPv6 block. This allowed large portions of the loopback block (e.g., 127.0.0.2), cloud metadata targets, and IPv4-mapped IPv6 equivalents to bypass validation. Furthermore, validation was not performed at execution, meaning if it was skipped or bypassed, the IP was fetched regardless, while correctly resolving public domains like 10.example.com were falsely flagged.

Fix: Replaced the ad-hoc string comparisons with a unified validateUrlInternal helper used in both the executor and validator. It strips trailing dots and leverages net.isIPv4 and net.isIPv6 to correctly parse dotted quads and bracketed IP ranges. It now explicitly matches the entire 127.0.0.0/8, 169.254.0.0/16, 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 networks, as well as equivalent IPv6 and IPv4-mapped formats. Since it restricts the range match to only valid IP addresses, it now correctly allows domains like 10.example.com.

Validation:

corepack enable >/dev/null 2>&1 || true
pnpm install --frozen-lockfile
pnpm run build
pnpm test:format
pnpm test:lint
pnpm test:types
pnpm test:knip
pnpm test:ava source/tools/fetch-url.spec.tsx

All commands ran and passed without errors, demonstrating validation rejects SSRF patterns and successfully compiles.

Fixes #1088

Fixes #1088