refactor(desktop): extract shared runtime/provider/model selection + env-var helpers (Phase 1B.1)

[wesbillman]

Phase 1B.1 — shared runtime/provider/model selection + env-var helpers

First slice of Phase 1B (UI unification) toward the unified Agent model (#1618 landed the record groundwork, #1623 folds the store). The persona, create-agent, and edit-agent dialogs each carried their own copy of the runtime → LLM provider → model dropdown state machine and the provider-API-key env-var bookkeeping. This PR extracts both into pure, tested modules and wires all three dialogs onto them — shrinking the three overlapping editors before B2 unifies them.

What moved

• providerEnvVarUpdates.ts (+10 tests): set/remove the managed provider API key, clear it on provider switch, merge Advanced-section env edits preserving the managed key. All helpers return the same reference on no-op.
• runtimeModelProviderSelection.ts (+12 tests): pure transition functions for the three dropdown handlers. Cross-dialog divergences are parameterized, never merged (per the reviewed extraction map):
  • lockedRuntimeReset: "full" (Persona/Edit clear the managed key + custom-model flag on switch to a provider-locked runtime) vs "provider-only" (Create keeps them).
  • clearModelWhenApiKeyMissing: Persona-only — provider switch clears the model when the new provider's API key isn't filled.
  • clearKnownModelOnCustomEntry: Persona-only — entering "Custom model..." drops a known model. When this flag is false (Create/Edit), the isModelCustom parameter is unused.
• Dialog-specific side effects stay at the call sites: Edit's inherit-pin/command-sync/advanced-auto-expand block, Create's catalog memory + command sync.

Behavior

Zero behavior change by design, with one disclosed exception: Create/Edit previously allocated fresh envVars objects on no-op deletes; the shared helpers return the same reference, removing spurious re-renders. (Persona already had these guards.)

Not adopted: useRequiredCredentialState in Persona/Create — the credential derivations are not equivalent (Persona's computeLocalModeGate falls back to the runtime file-config provider; Create/Edit don't), so per the map's conservative default nothing moved there.

Verification

• tsc, biome (full src/), file-size check: clean
• Frontend unit tests: 2056 passed / 0 failed (22 new)
• Smoke e2e -g "agent|persona": 56/56; full suite runs in CI
• Cross-reviewed by Pinky (independent verification run: same results), map-first review — approved

Part of the Phase 1B slice plan: each PR leaves the app fully working with both old and new paths mounted. Next: B2 (unified AgentDialog, create paths).