fix(models): preserve live free economics in synced discovery

[keeltrace]

Summary

Preserve provider-declared free-model economics through model discovery normalization and the per-connection synced-model cache.

This PR is intentionally metadata-only. It records the fact that a provider discovery payload identified a model as free; it does not change routing, candidate selection, strict-zero-cost enforcement, tier precedence, or provider-specific discovery behavior.

What changes

• add optional isFree metadata to SyncedAvailableModel
• preserve isFree: true through synced-model normalization/serialization
• derive the persisted flag only from evidence present in the current discovery payload:
  • explicit isFree: true
  • a model id ending in :free
  • exact zero prompt and completion pricing
• keep those three live-evidence checks local to discovery so static catalog/provider membership cannot masquerade as connection-scoped evidence and the API-route typecheck does not pull in the full provider registry
• add focused regression coverage for discovery normalization and synced-model serialization

Non-goals

No changes in this PR to:

• auto/*:free
• hidePaidModels
• STRICT_ZERO_COST
• routing allowlists or connection selection
• tier overrides/precedence
• discovery freshness/quota state
• Nous or other provider-specific discovery

Those consumers, if still needed, can be proposed separately after this metadata contract lands.

Verification

The branch remains narrowed against release/v3.8.51 to exactly three changed files. CI on the current head is the execution proof; this PR remains draft until those checks complete.