Providers show as configured when the user never set them up

🤖 Filed by Filip's AI agent on his behalf. Reported by @f1-outsourcing in discussion #11133 item 5 ("I have green check marks on providers I did not configure"), and independently in #11212.

**Describe the bug**

Providers show a green "configured" check on a fresh install. `default_inventory_configured` returns true when every required config key has a default, and also when a provider has no required keys at all (`crates/goose/src/providers/inventory/mod.rs:884-898`).

Affected: `github_copilot`, `claude_code`, `codex`, `gemini_cli`, `cursor_agent`, `bedrock`, `lmstudio`, `llama_swap`, `omlx`, `atomic_chat`. For example `CLAUDE_CODE_COMMAND` is required with default `"claude"` (`crates/goose/src/providers/claude_code.rs:600-604`), and the default is never validated, so the check appears whether or not the binary exists.

`ollama` and `openai` already override this with explicit `with_configured` resolvers, so per-provider semantics are treated as correct where someone bothered. The shared default is the problem.

**To Reproduce**

1. Fresh install, no providers configured
2. Settings -> Models -> Configure Providers
3. Green checks appear on providers that were never touched

**Expected behavior**

"Configured" means the user supplied configuration.

**Additional context**

The same flag filters the model picker (`ui/desktop/src/components/settings/models/subcomponents/SwitchModelModal.tsx:491`) and keeps deprecated providers on screen, so this is more than cosmetic.

#11100 fixes the ACP-adapter false positives, where PATH detection marked claude-acp, codex-acp and others as configured. It is unreleased and does not touch this default.

Blocks #11330: sorting configured providers to the top is only meaningful once this flag is accurate.