fix(desktop): fold baked build env into in-process model discovery

[wpfleger96]

Problem

Internal builds bake DATABRICKS_HOST (and other provider config) into the binary via BUZZ_DESKTOP_BUILD_AGENT_ENV. The baked pairs were applied only onto subprocess Commands via build_buzz_agent_provider_defaults. The in-process discovery path (discover_databricks_models, discover_openai_compatible_models, discover_anthropic_models) read config from merged_env + std::env::var — neither of which carries the baked value in a GUI-launched DMG (no inherited shell env).

The Edit Agent dialog also used a one-shot snapshot path (get_agent_models) that fired once on open keyed on the saved provider. This produced only "default model / custom model" for any agent whose provider was null or whose snapshot env lacked DATABRICKS_HOST.

These are the root causes of the v0.3.38 Databricks model dropdown regression.

Fix

Baked-floor discovery (commit 1)

Add baked_build_env() returning the baked pairs as a BTreeMap<String, String>, factored through build_env_map() so the assembly logic is unit-testable. In both get_agent_models and discover_agent_models, fold the baked map as a floor under merged_env before the discovery calls:

let merged_env = discovery_env_with_baked_floor(merged_env); // baked pairs = floor

Precedence (lowest → highest): baked build env → derived runtime env → user env_vars

OSS builds (all option_env! unset) return an empty map — no behavior change.

Live provider-driven discovery in Edit Agent dialog (commit 2)

Replace the snapshot discovery path with the same live, form-driven path the New Agent / Persona dialog already uses:

• EditAgentDialog: add provider + isCustomProviderEditing state seeded from the saved record. Derive selectedRuntime from the runtimes catalog via agentCommand. Wire usePersonaModelDiscovery so the model dropdown re-fires on every provider/env change without saving. Show EditAgentProviderField when the runtime supports LLM provider selection OR the agent already has a saved provider.
• UpdateManagedAgentInput (types.ts): add provider?: string | null with tri-state semantics.
• UpdateManagedAgentRequest (types.rs): add provider: Option<Option<String>> with double_option deserializer so absent/null/value are correctly distinguished at the Tauri boundary.
• update_managed_agent handler: apply provider tri-state alongside model.

Files Changed

• desktop/src-tauri/src/managed_agents/agent_env.rs — add baked_build_env() / build_env_map(), reimplement build_buzz_agent_provider_defaults, add 6 unit tests
• desktop/src-tauri/src/managed_agents/mod.rs — export baked_build_env
• desktop/src-tauri/src/commands/agent_models.rs — fold baked floor in both discovery commands; apply provider tri-state in update handler
• desktop/src-tauri/src/managed_agents/types.rs — add provider: Option<Option<String>> to UpdateManagedAgentRequest
• desktop/src-tauri/src/managed_agents/types/tests.rs — 3 new provider tri-state tests
• desktop/src/features/agents/ui/EditAgentDialog.tsx — live discovery via usePersonaModelDiscovery, provider dropdown, send provider on submit
• desktop/src/shared/api/types.ts — add provider?: string | null to UpdateManagedAgentInput
• desktop/src/features/agents/ui/editAgentProviderDiscovery.test.mjs — 8 new JS tests