feat(wrapper-ts): add listAllModels() for aggregate provider discovery

[manojp99]

Summary

Add a new listAllModels() function to the TypeScript wrapper, exposing the engine's aggregate provider discovery mode introduced in #49.

• Queries all configured providers in parallel (anthropic, openai, ollama, azure-openai)
• Returns per-provider results with auth status for each
• Eliminates the need for hosts to make 4 sequential or parallel single-provider calls

Motivation

The paperclip adapter currently uses a static list of models when selecting which provider/model to use. The ideal flow is:

1. Call listAllModels() once to get all available models across all configured providers
2. Check auth status per-provider (ok vs credentials_missing vs module_not_installed)
3. Use a single dynamic dropdown instead of hardcoded options

With the engine's existing aggregate mode (amplifier-agent models list with no --provider flag), this is now efficient and straightforward.

Design

Separate function, not overload:

• New listAllModels() is a distinct exported function returning a different envelope type
• Existing listModels(provider) behavior is unchanged and fully backward compatible
• Union return types would complicate call-site type narrowing; distinct functions are clearer

Status is a free-form string:

• Engine emits ok | credentials_missing | module_not_installed | error today
• Wrapper accepts any string to permit future engine additions without wrapper version bumps
• Hosts should compare against literal "ok" rather than exhaustive enums

Refactored helpers for code reuse:

• Extracted runModelsListSubprocess(args, options) and interpretOutcome(outcome, validate)
• Both listModels and listAllModels share the same subprocess driver + exit-code interpretation
• No behavioral changes to existing listModels() callers

Testing

• 11 new test cases (A1-A11) mirror the existing 9 for single-provider mode
• Additional tests for the aggregate-specific scenarios (mixed provider statuses, missing results[] field)
• All 124 tests pass (pnpm test)
• Asserts correct argv construction (omits --provider in aggregate mode, includes it in single-provider)

Engine compatibility

Requires engine ≥ #49 (merged into main at commit e48a644). The aggregate CLI endpoint is already live — no engine changes in this PR.

Backward compatibility

Zero breaking changes. Existing listModels(provider) callers see no change in signature, behavior, or error types.