test(provider): gate user-facing strings against the legacy provider names

[code-yeongyu]

Turns "no leaked old name" into an enforced gate. Part of #1989 (todo 11). Rebased onto main now that #2018 (todo 10) has merged; the gate is verified 3/3 against the post-rename tree.

(Reopened from #2019, whose base branch was deleted on #2018's merge — that froze its head at the pre-rebase sha and GitHub will not retarget a PR with a deleted base ref.)

How it works

Scans every shipped .ts under packages/coding-agent/src and packages/ai/src, extracts string and template literals only (comments and identifiers are not user-facing), and fails on any literal carrying openai-codex, OpenAI Codex, claude-sdk-oauth or Claude SDK OAuth outside the allowlist.

The allowlist is (file, literal) PAIRS

Not whole files — a new legacy string in an already-listed file still fails. Each entry states why it is frozen:

File	Literal	Why
anthropic-subscription/api-id.ts	claude-sdk-oauth	frozen wire api id (its own comment: "Frozen permanently — must NOT follow a provider-id rename")
goal/terminal-provider-error.ts, utils/prompt-cache-ttl.ts	claude-sdk-oauth	compare/switch on model.api, the frozen wire id
anthropic-subscription/affinity.ts	claude-sdk-oauth-default	persisted HRW affinity key
anthropic-subscription/session-binding.ts	claude-sdk-oauth-binding	persisted binding entry type
anthropic-subscription/config-dir-credentials.ts	claude-sdk-oauth-accounts	legacy accounts dir the one-shot move reads from
extensions/builtin/index.ts	claude-sdk-oauth	builtin extension module id
packages/ai/src/types.ts	openai-codex	retained KnownProvider legacy member

Allowed anywhere because the string is the contract: the seven frozen tokens, CLAUDE_CODE_OAUTH_TOKEN/SENPI_CLAUDE_SDK_OAUTH/claude_sdk_oauth_, and "was renamed to" — the todo-9 rejection message must name the old id back to the user.

Allowed wholesale because knowing the legacy ids is their entire job: legacy-provider-ids.ts, legacy-api-aliases.ts, auth-provider-key-migration.ts, settings-manager.ts.

Scope decisions worth reviewing

• Both casings, both providers. An earlier symbol inventory covered SCREAMING_SNAKE only on the Anthropic side and missed OPENAI_CODEX_*; a one-sided gate would be blind to half its surface.
• Excludes changes.md/AGENTS.md. 166 of 201 legacy hits in the tree are dated design-history entries; a gate demanding those be rewritten would attack the fork's own record.
• A third case pins the load-bearing direction: both LEGACY_PROVIDER_IDS map keys must stay reachable, since every normalization and every typed rejection reads them.

Verification

bunx vitest run test/suite/anthropic-subscription-naming.test.ts — 3 passed, run on the rebased head whose base contains the full rename.

Proven live, as the plan requires: re-introducing "OpenAI Codex" as a display name in provider-display-names.ts turns the guard RED naming the offending file and literal:

+   "packages/coding-agent/src/core/provider-display-names.ts: OpenAI Codex",

Restored afterwards; 3 passed again. Biome 0 errors; changelog gate PASS (test-only diff).

Observation for a follow-up, not fixed here

extensions/builtin/index.ts still registers { id: "claude-sdk-oauth" } while todo 10 moved its directory to anthropic-subscription/, so the builtin id no longer mirrors its directory. Keeping it is correct for a behaviour-free commit (it is a stable module identifier), but whether it should be renamed with a migration deserves its own issue.

Part of #1989

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Summary by cubic

Adds a test that makes "no leaked old provider name" an enforced gate across packages/coding-agent/src and packages/ai/src.

• Scans only string and template literals (comments and identifiers are not user-facing) and fails on any carrying openai-codex, OpenAI Codex, claude-sdk-oauth, or Claude SDK OAuth.
• Allowlists are per literal (file + literal pairs), not whole files, so a new legacy string in an already-listed file still fails; each entry documents why it is frozen (wire API ids, persisted tokens, stable module ids, retained union member).
• Allows frozen contract substrings anywhere (persisted tokens and env vars) and allows four files wholesale whose job is to know the legacy ids.
• Adds a separate assertion that both LEGACY_PROVIDER_IDS map keys stay reachable since every normalization and rejection reads them.

Part of #1989.

^{Written for commit c947b4a. Summary will update on new commits.}