Fix model provider filter labels - #1471

Why

The Models page grouped filter options by Model.provider but rendered each provider value through AgentBadge as if it were an agent ID. Provider names therefore failed agent lookup and appeared as duplicate Unknown agent entries, making the filter ambiguous and unreliable.

What changed

• Build provider filter options directly from the exact Model.provider values, preserving distinct labels, counts, and URL filter values.
• Keep Model.agentId exclusively for the separate Agent filter and table badge.
• Remove the redundant agent catalog request that was only used for the incorrect provider label lookup.
• Add focused unit coverage for distinct provider labels, deterministic sorting, and grouped counts.
• Document the Models page provider-versus-agent filter semantics.

Validation

• pnpm test apps/portal/src/pages/model-provider-filter-options.test.ts - passed, 2 tests
• pnpm --filter portal lint - passed
• pnpm --filter portal build - passed

Walkthrough

Fixes: #1469