fix: switch active provider on /login and /logout

[assapir]

Problem

When logged into both Anthropic and Google, /model always queries the Anthropic provider regardless of which provider was last authenticated via /login. After /logout from the current provider, /model still tries the old provider even when another is authenticated. The startup banner only showed the active provider's auth status, with no visibility into other authenticated providers.

Fixes #58

Root Cause

/login to a different provider returned CommandResult::Handled (no-op) instead of switching the active provider. The REPL's thinker was never swapped. /model only queried the single active provider.

Changes

1. Provider switching on /login and /logout

StateChange::Provider(String, Option<String>) — new variant that tells the REPL to switch provider, with an optional model override.

build_provider_by_id(id, db_path, model, debug) — builds a ProviderSetup from a string id. Used for runtime provider switches.

/login — when login succeeds for a different provider, returns StateChange::Provider to switch to it. Auth status is derived from storage (not hard-coded).

/logout — after logging out from the current provider, falls back to another authenticated provider if one exists (checks both stored credentials and env vars).

REPL handler — handles StateChange::Provider by calling build_provider_by_id(), swapping the engine's thinker, updating all display state, and persisting the new model preference to the config DB.

2. Banner shows all providers

all_provider_statuses(db_path) — returns auth status for every configurable provider.

The startup banner now lists every provider with its auth status, marking the active one:

   auth      Anthropic (Claude Pro/Max) (OAuth ✓) ← active
             Google (Gemini) (not authenticated)

3. /model shows models from all authenticated providers

/model now fetches models from all authenticated providers (not just the active one), displays them grouped by provider, and allows switching provider + model in one step:

  Anthropic (Claude) ← active:
    1. Claude Sonnet 4 ← current
    2. Claude Haiku 3.5

  Google (Gemini):
    3. Gemini 2.0 Flash
    4. Gemini 2.5 Pro

  Select model [1]:

Selecting a model from a different provider automatically switches the active provider via StateChange::Provider(id, Some(model)).

4. Review feedback addressed

• Removed unnecessary env var save/restore from build_provider_by_id tests (avoids parallel test races)
• Logout fallback checks env vars too, not just stored credentials (consistent with rest of app)
• Login derives auth status from storage via provider_auth_status() instead of hard-coding "OAuth ✓"
• Provider switch persists model preference to config DB

New public API

Function	Purpose
build_provider_by_id()	Build a provider by string id with optional model override
all_provider_statuses()	Auth status for all providers (for banner)
provider_auth_status()	Auth status for a single provider
is_authenticated()	Bool check for provider authentication

Testing

23 new tests covering:

• build_provider_by_id — known/unknown providers, auth detection, default model, model override
• find_authenticated_provider — fallback logic (empty, excluded, found)
• StateChange::Provider — variant dispatch
• all_provider_statuses / is_authenticated / provider_auth_status — status helpers
• find_other_authenticated_providers — empty db
• Banner — multiple providers, no providers
• provider_display_name — known and unknown ids

[Copilot]

Pull request overview

This PR fixes REPL provider state not updating when authenticating/logging out across multiple providers, ensuring commands like /model operate against the currently active authenticated provider.

Changes:

• Added StateChange::Provider(String) to allow commands to request a provider switch.
• Introduced build_provider_by_id() for rebuilding the active provider/thinker using provider defaults when switching at runtime.
• Updated /login and /logout flows plus the REPL handler to switch providers appropriately; added tests for the new behavior.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/login_test.rs	Adds tests for build_provider_by_id behavior and auth/model resolution expectations.
src/provider.rs	Adds build_provider_by_id() helper for runtime provider switching using default model.
src/main.rs	Handles StateChange::Provider by rebuilding provider setup and swapping the engine thinker + display state.
src/commands/mod.rs	Adds StateChange::Provider and a registry dispatch test to ensure the payload is carried through.
src/commands/logout.rs	Adds fallback logic to switch to another provider after logout if credentials exist.
src/commands/login.rs	Updates /login to switch active provider when logging into a different provider.

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