fix(desktop): let explicit model switches escape broken config providers (salvage #42971)

[austinpickett]

Summary

Salvage of #42971 (@rodboev) onto current main. Fixes #42241.

When a desktop/dashboard session had no agent built yet and the user explicitly picked a provider in the model picker, config.set('model', …) would first try to initialize the agent from the (possibly broken) config default provider — failing before the user's explicit switch could take effect. The user got trapped on a misconfigured default with no way to switch out via the UI.

Confirmed still present on main: tui_gateway/server.py _apply_model_switch() else-branch unconditionally calls resolve_runtime_provider(requested=None), and the config.set model handler does _start_agent_build whenever session.get("agent") is None regardless of an explicit provider.

What changed

• config.set pre-parses the model flags. If an explicit --provider is present and no agent exists yet, it skips the default-provider agent build and routes straight through _apply_model_switch with the explicit provider.
• _apply_model_switch gained a parsed_flags passthrough (avoids double-parsing) and only falls back to resolve_runtime_provider(requested=None) when no explicit provider was given; otherwise it uses the explicit provider directly.
• The desktop hook (use-model-controls.ts) sends config.set instead of slash.exec for active-session changes, so errors from the selected provider surface to the user instead of being swallowed.
• Adds 2 backend tests (explicit provider skips broken default init; selected-provider errors surface as code 5001) and expands the desktop hook tests.

How this was salvaged

The original PR was 4 commits (core fix + i18n test alignment + a refactor that removed 26 lockfile lines + a follow-up that re-added them — net lockfile change zero). I applied the net diff of the 4 real files via 3-way merge onto current main. One conflict in _apply_model_switch's signature — main added a pin_session_override param since the PR was authored; kept both that and the new parsed_flags param. Dropped the no-op lockfile churn entirely.

Test plan

• py_compile on server.py → OK
• pytest tests/test_tui_gateway_server.py -k model → 20 passed (incl. the 2 new tests + the env-leak guard)
• Desktop vitest changes applied cleanly (deps not installed locally; CI runs the desktop suite)

Credit

• fix(desktop): let explicit model switches escape broken config providers (#42241) #42971 by @rodboev — authorship preserved on the commit

[github-actions]

🔎 Lint report: salvage/model-switch-escape-42971 vs origin/main

ruff

Total: 0 on HEAD, 0 on base (➖ 0)

🆕 New issues: none

✅ Fixed issues: none

Unchanged: 0 pre-existing issues carried over.

ty (type checker)

Total: 10940 on HEAD, 10931 on base (🆕 +9)

🆕 New issues (4):

Rule	Count
unresolved-attribute	3
unsupported-operator	1

First entries

tests/test_tui_gateway_server.py:3051: [unresolved-attribute] unresolved-attribute: Attribute `append` is not defined on `int` in union `int | list[Unknown]`
run_agent.py:2920: [unresolved-attribute] unresolved-attribute: Object of type `Self@get_credits_spent_micros` has no attribute `_credits_session_start_micros`
tests/test_tui_gateway_server.py:3046: [unsupported-operator] unsupported-operator: Operator `+` is not supported between objects of type `int | list[Unknown]` and `Literal[1]`
tests/run_agent/test_credits_notices_toggle.py:76: [unresolved-attribute] unresolved-attribute: Unresolved attribute `_credits_session_start_micros` on type `AIAgent`

✅ Fixed issues (1):

Rule	Count
invalid-assignment	1

First entries

tests/run_agent/test_credits_notices_toggle.py:76: [invalid-assignment] invalid-assignment: Object of type `None` is not assignable to attribute `_credits_session_start_micros` of type `int`

Unchanged: 5758 pre-existing issues carried over.

Diagnostics are surfaced as warnings — this check never fails the build.

[alt-glitch]

Related: #42971 (original by @rodboev — this is the salvage onto current main), #42241 (the issue it fixes), and #46288 (sibling desktop model-switch fix via a different mechanism — routing picker switches through config.set to avoid the slash worker). This salvage is the active/authoritative version of the #42971 fix; tracking lineage only.