feat(router): same-model cross-provider fallback

[stephenzwj]

What it does

When a client pins an explicit model and that same model_id is served by multiple enabled platforms, failed attempts now try sibling rows for the same model immediately after the pinned row, before falling through to unrelated models.

Previously, a failed explicit pin could fall through to whichever unrelated model scored next in the chain.

Scope

• Request-scoped: only applies when preferredModelDbId is set.
• Auto and sticky routing behavior is unchanged.
• No-op when the pinned model has no same-model siblings.

Tests

Adds four regression tests covering:

• pinned model remains first;
• same-model sibling is selected before an unrelated model;
• unrelated model is selected after all same-model rows are exhausted;
• no-sibling behavior remains unchanged.

Full server test suite: 214 files, 2,352 tests passed, 5 skipped.
Server build passes.