backends: reverse-geocode config and the state-room exclusion list are copied across packages #2492

Context

Small pieces of configuration and protocol logic are copied across packages with no shared source:

1. Reverse-geocode configuration. resolveLocationReverseGeocodeConfig is identical in packages/ui-backend-claude/src/location-tool.ts:18-26 and packages/ui-backend-pi/src/bridge-tools.ts:74-82. The env descriptors behind it are also duplicated, including the literal defaults: BRAIN_UI_REVERSE_GEOCODE, the three NOMINATIM_* variables, "https://nominatim.openstreetmap.org" and "brain-kit-ui/1.0" (packages/ui-backend-claude/src/config/env.ts:128, :172; packages/ui-backend-pi/src/config/env.ts:117, :139). If one default changes, the two backends geocode differently.
2. State-room exclusion list. The rule "attach stateRoom to a result unless failure.errorClass is worker_state_busy, session_too_large or worker_state_exceeded" is the same inline string array in three packages: packages/ui-backend-claude/src/turn-runner.ts:217, packages/ui-backend-pi/src/turn-runner.ts:81 and packages/ui-react/src/hooks/websocket-handlers/chat.ts:383. The protocol already exports the class names as FIRST_PARTY_FAILURE_CLASSES (packages/ui-sdk/src/protocol.ts:1201).

git grep -n '"worker_state_busy", "session_too_large", "worker_state_exceeded"' -- packages ':!**/tests/**'
git grep -n 'function resolveLocationReverseGeocodeConfig' -- packages

Scope

• Export stateRoomEligible(failure), or an equivalent constant set built from FIRST_PARTY_FAILURE_CLASSES, from @schlessera/brain-ui-sdk/protocol. Use it at the three sites.
• Export the reverse-geocode env descriptors and resolveReverseGeocodeConfig(env) from @schlessera/brain-ui-sdk/server, which already owns ReverseGeocodeConfig. Each backend's env chokepoint spreads the descriptors and passes its own process.env read, so the "only file that reads process.env" rule still holds. Delete both resolveLocationReverseGeocodeConfig copies.

Out of scope

• resolveExecConfig in both backends. backends: resolve the exec wrapper from the host's app config instead of process.env #1429 is reworking it (draft PR CONTRACT: fix(backends): use the host app exec wrapper #2397); fold it in only after that lands.
• The two SessionTooLargeError classes (packages/ui-backend-claude/src/worker-state.ts:23, packages/ui-backend-pi/src/worker-protocol.ts:12). They share an error class, but their constructors and messages differ on purpose.
• EnvVarSpec redeclarations in seven packages.
• Behaviour or default-value changes.

Acceptance criteria

• The first grep above returns no src/ hits; the three sites use the shared helper.
• git grep -n 'brain-kit-ui/1.0' -- packages/ui-backend-claude/src packages/ui-backend-pi/src returns nothing.
• A ui-sdk unit test covers the state-room rule for each first-party failure class, and one covers resolveReverseGeocodeConfig defaults and overrides.
• Both backends' location-tool tests and the ui-react state-room tests pass unchanged.

Verification

bun run test packages/ui-sdk packages/ui-backend-claude packages/ui-backend-pi packages/ui-react
bun run api-report
bun run env-docs
bun run typecheck

Contract impact

Additive @experimental exports on @schlessera/brain-ui-sdk/protocol and /server: CONTRACT: commit, with the API report and contract updated. Env variable names and defaults are unchanged.

Found by a repository review on 2026-10-22.