chore: split credentials.tsx pure logic into credentials/ modules with tests

[Colin Francis (colifran)]

Summary

Refactor phase 3, following #611: this breaks the ~5,700-line credentials.tsx file into focused, individually testable modules under src/setup/credentials/, adds coverage for the extracted units, and fixes one setup-flow bug the new tests surfaced.

credentials.tsx is now a thin composition root (30 lines): the setup wizard's state machine lives in a useInitSetup hook, its rendering in an InitSetupView, and its pure logic (routing, formatting, env persistence) in sibling modules with their own test files. The extraction is mechanical and behavior-neutral, with one intentional exception called out under Behavior change below, so reviews get smaller and regressions in the first-run setup surface get caught.

Structure

credentials.tsx went from ~5,744 lines to 30 (a composition root + public re-export shim). Its contents were extracted into:

src/setup/credentials/
  use-init-setup.ts   Ink useInput state machine (the controller)
  view.tsx            presentational InitSetupView
  components.tsx      leaf components + the Prompt step router
  steps.ts            pure step routing / spine / status logic
  format.ts           credential display formatting + masking
  persistence.ts      pure buildCredentialEnvUpdates (env-write derivation)
  constants.ts        types.ts

test/setup/credentials/ mirrors this layout, so every extracted pure module has a sibling test. The use-init-setup.ts controller is coverage-excluded (documented in vitest.config.ts): it is a keyboard-driven useInput flow that unit tests cannot drive cleanly, and its extractable logic already lives in the tested modules above.

Coverage

Both columns measured under the repo's all: true vitest config.

Metric	Before (main)	After
Statements	70.97%	87.74%
Branches	64.61%	82.45%
Functions	78.43%	92.62%
Lines	71.00%	87.74%
Test files / tests	131 / 1751	137 / 2035

Behavior change

The optional LangSmith setup step no longer re-prompts after it has been declined. needsCredentialSetup already gated the step on the two-signal needsLangSmithStep() (a recorded LANGCHAIN_TRACING_V2 decision counts as answered), but the two entry-point routers — getInitialStep and getNextStepAfterRegion — still used a naive !process.env.LANGSMITH_API_KEY check, so a user who declined LangSmith got sent back into the step whenever setup reopened for another reason. Both routers now use needsLangSmithStep(), with a decision-table regression test covering both routers across all four env states.

Testing

New unit tests for every extracted pure module plus the onboarding.ts gaps above. All tests pass (2035/2035). Typecheck, lint, and build are clean.

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Latest commit: 7345805

The changes in this PR will be included in the next version bump.

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