Fix desktop model switcher provider scope merge / 修复桌面模型切换器配置作用域合并

[SivanCola]

Summary

• Merge [[providers]] across user and project TOML sources by provider name, matching the existing plugin merge behavior.
• Merge desktop provider_access across config scopes so project desktop settings do not hide account-level providers from the chat model switcher.
• Add a regression test for a workspace project config that previously hid user-configured MiniMax models.

Fixes #4486.

Verification

• go test ./internal/config
• go test ./internal/config ./internal/control ./internal/boot with isolated user config
• go test . in the desktop module

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[SivanCola]

Follow-up pushed in 33cf89d2 to fix the CI failures from the provider merge follow-up.

Root cause:

• The previous follow-up merged official provider aliases by canonical Settings ID, which collapsed distinct legacy providers such as deepseek-flash and deepseek-pro into one entry.
• That broke legacy model refs and effort capability lookup, causing the failing internal/boot, internal/cli, and internal/control tests.

Fix:

• Merge provider tables by provider name again, keeping official legacy aliases distinct.
• Keep the project-render scoping behavior so merged user providers are not written back into project config files.

Verified locally:

• go test ./internal/boot -run TestBuildMigratesLegacyConfigEndToEnd -count=1
• go test ./internal/cli -run TestEffortCommandAutoClearsProviderEffort -count=1
• go test ./internal/control -run TestSlashArgItems -count=1
• go test ./internal/config -run 'TestLoadForRootKeepsOfficialProviderAliasesDistinct|TestSaveForRootDoesNotWriteUserProvidersIntoProjectConfig' -count=1
• go test ./internal/config ./internal/boot ./internal/cli ./internal/control with isolated user config
• go test -race ./internal/boot ./internal/cli ./internal/control with isolated user config
• go test . in the desktop module

GitHub CI is now green on the updated head.