[yoruuuchan]

Summary

• control.Accounts.ImportAll resolves a credential importer with adapter.Credential.(providers.CredentialImporter), but the CodeArts adapter declared Format and PrepareImport on importer{} — the value in Adapter.ImportExport. The type assertion never matched, no provider claimed the format, and the loop returned its final unsupported_format.
• The console's 导入凭证 / Import credential tab is therefore broken for 码道: pasting a valid codearts-oauth-v1 credential answers 400 unsupported_format, and no account is created. Browser OAuth login is unaffected, which is why this stayed hidden.
• Every other in-process provider puts these two methods on its credential codec (kimi/import.go:12, and qwenwork asserts the same in its own test), so this PR moves them onto credentialCodec and leaves ImportExport holding ValidateImport + Export only.

Reproduced against the published image, same request body, only the server differs:

POST /api/accounts/import  {"format":"codearts-oauth-v1","provider":"codearts","region":"cn-north-4","credential":{...}}

ghcr.io/caigee-cmd/cli2api:latest (v0.7.26)  -> 400 {"code":"unsupported_format"}
this branch (built from cmd/server)          -> 201 {"id":"acc_...","provider":"codearts"}

Test plan

• go test ./internal/providers/... ./internal/control/... ./internal/console/... ./internal/app/... ./internal/gateway/... — green
• go vet ./... — green
• git diff --check — clean
• python scripts/release-notes.py validate — green; added changelog/unreleased/codearts-credential-import.md
• No frontend or worker change, so no npm run sync / worker test needed
• New coverage: TestS04InterfaceContract now asserts the Adapter.Credential slot itself implements providers.CredentialImporter, and TestCredentialCodecServesConsoleImport drives Format + PrepareImport on an encoded credential — the two halves the console consumes
• No tokens, auth blobs, raw captures, or host details in the diff

Note for local runners: on my Windows box a clean fb2eb35 checkout already fails TestLoadLocalCredentialMissing, TestRuntimeSpecSelectsCNCLIAndConfigDir, TestStarterEnvSetsRegionAndProxy, TestRepeatedFailuresBackOff, TestBackoffResetsOnKindChange, and the internal/store backup/tempdir cases (backup mode = 666, want 600, locked-file cleanup). Verified by running the same packages in a worktree at fb2eb35. CI does not reproduce them, so it is local-environment noise, not this PR.

Scope

Only the import wiring changes. The ImportExport descriptor capability, the export path's ErrUnsupported answer, PrepareImport's validation, and the OAuth login flow are untouched. The changelog/unreleased/codearts-provider.md fragment from the original provider PR has already shipped in v0.7.26, so this uses a new filename.