fix(mcp): allow configured private network endpoints

[YOMXXX]

Summary

• allow explicitly configured remote MCP servers to use private RFC1918/ULA endpoints such as Docker bridge addresses
• keep WebFetch/default SSRF checks strict and continue blocking metadata/link-local endpoints
• add regression coverage for Docker bridge MCP config and scoped private-network URL validation

Fixes #1315

Tests

• bun test test/util/ssrf.test.ts --timeout 30000
• bun test test/mcp/lifecycle.test.ts --timeout 30000 --test-name-pattern "remote MCP can connect"
• bun test test/mcp/headers.test.ts --timeout 30000
• bun typecheck
• git diff --check

Note: full bun test test/mcp/lifecycle.test.ts --timeout 30000 still has the pre-existing dev-branch failure Claude Code local MCP server is pending until explicitly connected (expected pending, actual connected); the new Docker bridge regression passes.