fix(setup): restore sibling built-in entries when re-running setup wizard

[I-personal-account]

Bug Description

After selecting a DeepSeek model using reasonix setup (e.g., selecting only deepseek-v4-flash),
running reasonix setup again causes the model selection list to display only the previously selected model;
other available models (such as deepseek-v4-pro) are not visible.

Root Cause

withBuiltinFamiliesForLanguage performs deduplication based on the family key: since the user already has deepseek-flash
(which belongs to the deepseek family), the entire family is considered "already present." Consequently, the built-in deepseek-pro
entry is not added, leaving the static model fallback list containing only the single previously selected model.

Fix

Change the deduplication logic from by family key to by entry name:

• Existing user entries with the same name (e.g., a custom deepseek-flash) remain unchanged.
• Missing built-in entries (e.g., deepseek-pro) are automatically added.

This ensures familyStaticModels returns the complete list of models.

Testing

• Added a new regression test: TestWithBuiltinFamiliesRestoresSiblingEntries
• Existing test behavior remains unchanged.

Bug 描述

使用 reasonix setup 选择了一个 DeepSeek 模型后（如只选了 deepseek-v4-flash），
再次运行 reasonix setup 时，模型选择列表只显示之前选过的模型，
无法看到其他可选模型（如 deepseek-v4-pro）。

根因

withBuiltinFamiliesForLanguage 按 family key 去重：用户已有 deepseek-flash
（属于 deepseek family），整个 family 被认为"已存在"，导致内置的 deepseek-pro
条目不会被补充，模型静态回退列表只有之前选过的一个模型。

修复

将去重逻辑从 按 family key 改为 按 entry name：

• 用户已有的同名条目（如自定义的 deepseek-flash）保持不变
• 缺失的内置条目（如 deepseek-pro）会被自动补充

这样 familyStaticModels 就能返回完整的模型列表。

测试

• 新增回归测试 TestWithBuiltinFamiliesRestoresSiblingEntries
• 现有测试行为不变

[SivanCola]

Approved. This is a well-scoped fix. The PR corrects setup re-run model selection by changing built-in family supplementation from family-key dedupe to exact provider-name dedupe, so a saved deepseek-flash entry no longer hides deepseek-pro. Existing same-name user entries remain preserved, and buildFamilyEntries continues to keep per-SKU pricing/metadata after selection.

Reviewed:

• Baseline: latest main-v2 and PR head diff; mergeability is clean.
• Related PRs: #4518 touches the same file in chatREPL only; no conflict with setup provider logic.
• Security/cache: no new I/O, network calls, dependencies, prompt/tool schema changes, or provider request serialization changes.
• Tests: go test ./internal/cli -count=1 -run 'TestWithBuiltinFamilies'; go test ./internal/config -count=1 -run 'TestBackfillDeepSeekPro|TestNormalizeLegacyProviderModels|TestResetOfficialProviderPricingOnUpgradeRunsOnce'; git diff --check origin/main-v2...origin/pr/4673; CI is green.