feat: MongoDB replica set multi-host connections

[datlechin]

Summary

• Add multi-host connection support for MongoDB replica sets (closes MongoDB replica set: no support for multiple hosts #854)
• New HostListFieldRow UI component with add/remove host:port rows in the connection form
• New FieldType.hostList and FieldSection.connection in PluginKit for generic multi-host field support
• Custom URL parser for multi-host MongoDB URIs (mongodb://h1:27017,h2:27017,h3:27017/db)
• Backward compatible: existing single-host connections auto-migrate in the form

Changes

PluginKit — FieldType.hostList, FieldSection.connection (no ABI bump needed)

MongoDB Plugin — mongoHosts field, URI builder handles comma-separated hosts, driver prefers mongoHosts from additionalFields

Connection Form — HostListFieldRow multi-row editor, host/port derived from first entry on save, legacy connection migration on load, SSH + multi-host warning

URL Parser — parseMultiHostMongoDB() for proper authority parsing without URLComponents, multiHost field on ParsedConnectionURL

Display — hostDisplayString on DatabaseConnection shows host1:27017 (+2 more) for multi-host

Docs — Replica Set / Multi-Host section in docs/databases/mongodb.mdx

Test plan

• Create new MongoDB connection with single host — verify host/port fields work as before
• Add multiple hosts via "Add Host" button — verify all entries saved
• Remove hosts — verify minimum 1 row enforced
• Save multi-host connection — verify primary host/port derived from first entry
• Edit existing single-host MongoDB connection — verify mongoHosts synthesized from host:port
• Import URL mongodb://h1:27017,h2:27017,h3:27017/db?replicaSet=myrs — verify all hosts populated
• Import URL mongodb://user:pass@host1:27017/db — verify single host works
• Import URL mongodb+srv://cluster.mongodb.net/db — verify SRV still works (no multi-host path)
• Verify welcome screen shows host1:27017 (+2 more) for multi-host connections
• Enable SSH with multi-host — verify warning message appears
• Test connection with multi-host — verify connection succeeds
• Switch database type away from MongoDB and back — verify form resets correctly