fix(tui): reject stale session model switches

[Success6666]

Summary

• reject config.set updates when a non-empty session_id no longer maps to a live gateway session
• keep no-session calls available for intentional global/default config updates
• add regression coverage so stale desktop/TUI session requests cannot fall through to key-specific global paths
• keep the fix scoped to the gateway guard while validating the existing desktop model controls path

Fixes #56058

Tests

• uv run --extra dev pytest tests/test_tui_gateway_server.py -k config_set -q
• npm --workspace apps/desktop run test:ui -- src/app/session/hooks/use-model-controls.test.tsx
• uv run --extra dev ruff check tui_gateway/server.py tests/test_tui_gateway_server.py
• git diff --check origin/main...HEAD

[Copilot]

Pull request overview

This PR fixes a TUI gateway edge case where config.set model changes could incorrectly fall back to the global/default model switch path when the caller provided a stale session_id that no longer maps to a live session. This aligns desktop/TUI model picker behavior with the intended “session-scoped override unless explicitly global” contract.

Changes:

• Reject config.set requests for key=="model" when a non-empty session_id is provided but not found in _sessions.
• Preserve the intentional global model-switch behavior when session_id is empty/omitted.
• Add a regression test ensuring stale session model switch requests do not hit the global model switch path.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tui_gateway/server.py	Rejects stale session_id model updates instead of falling back to global model switching.
tests/test_tui_gateway_server.py	Adds coverage to prevent missing-session requests from invoking the global model switch path.

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[Success6666]

Update: rebased this branch onto current main and reran the focused validation.

Validation:

• uv run --extra dev pytest tests/test_tui_gateway_server.py -k config_set -q — 32 passed, 272 deselected
• npm --workspace apps/desktop run test:ui -- src/app/session/hooks/use-model-controls.test.tsx — 1 file passed, 5 tests passed
• uv run --extra dev ruff check tui_gateway/server.py tests/test_tui_gateway_server.py — passed
• git diff --check origin/main...HEAD — passed

Current head: 91db1d4862626490bc5080e8285b1148891371ad.

[teknium1]

Thanks for the focused regression fix. Current main still has the reported fallthrough: tui_gateway/server.py:10243 looks up the requested session without distinguishing a supplied stale ID from an omitted ID, and tui_gateway/server.py:10285-10293 routes that case through _apply_model_switch("", {"agent": None}, ...). _apply_model_switch() can persist the resolved model globally (tui_gateway/server.py:2852-2855, 3007-3008).

The PR's early 4001 guard distinguishes those cases while retaining the intentional empty-ID global path, and its added regression covers model plus the other mutating config.set keys. This is an automated hermes-sweeper review.

[Success6666]

Rebased the branch onto the latest main and pushed the updated head 63f0fdc. The patch remains limited to rejecting stale session-scoped config.set model updates while preserving the explicit global path.\n\nValidation on the rebased head:\n- uv run --extra dev pytest tests/test_tui_gateway_server.py -k config_set -q - 34 passed, 290 deselected\n-
pm --workspace apps/desktop run test:ui -- src/app/session/hooks/use-model-controls.test.tsx - 6 tests passed\n- focused Ruff check - passed\n- git diff --check origin/main...HEAD - passed