fix(providers): use mapped Sonnet model for connection tests

[NickyLam]

Summary

• Connection tests for third-party Anthropic providers now send the user-mapped Sonnet model when no explicit default model is set, instead of falling back to the catalog alias sonnet.
• Mapping-only presets no longer seed a hidden modelName from defaultModelId, so that alias cannot override the Sonnet mapping in the test payload.
• Presets that expose model_names (for example MiMo) still pre-fill the editable model field, preserving #577A behavior.
• Adds a small pure helper and unit coverage for the resolution order and the dialog wiring.

Root cause

PresetConnectDialog only sent a single modelName on connection test. Create-mode pre-fill wrote preset.defaultModelId (often the catalog alias sonnet) into that field even when the field was not shown. Users who only filled Sonnet mapping therefore still probed with model: "sonnet", while chat could already use the mapped upstream id.

Why not only #575

Open PR #575 also tries to prefer role mappings, but if the hidden default still pre-fills modelName with sonnet, that value wins before mapSonnet. This PR closes that gap with a narrower change and without the 503 classifier expansion.

Verification

• Node 22 (v22.23.2)
• npx tsc --noEmit — pass
• Targeted: provider-connection-test-model, mimo-model-mapping, provider-key-lifecycle, provider-preset — 144 pass / 0 fail
• Full unit suite (excluding pre-existing hang in native-timeout-reasons.test.ts): 4507 pass / 11 fail
  • All 11 failures are pre-existing provider-request-shape Anthropic tests polluted by a local default endpoint (ark.cn-beijing.volces.com), unrelated to this patch
• Prior session browser smoke: intercepted request used the mapped Sonnet model and reported a successful connection

Related

Related: #461

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