test: align coverage with public seams

[Waishnav]

Follow-up to #265. A few tests still exercised internal helper structure, live environment state, or exact internal representations rather than behavior at the seam callers actually use. Those checks made refactors noisier without protecting meaningful DevSpace behavior.

This keeps coverage at public or external boundaries: provider availability is verified through the production snapshot API, CLI-facing structured presentation remains covered without freezing formatter helpers, and security-sensitive provider command/sandbox mappings stay intact.

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[greptile-apps]

Greptile Summary

This PR realigns tests around externally observable behavior and makes the provider availability checker module-private.

• Verifies provider availability through the production snapshot API.
• Removes assertions tied to runtime-key, formatter, encoder, onboarding-helper, and default-representation internals.
• Preserves coverage for provider command and sandbox mappings.
• Consolidates card expansion tests around the shared expandability behavior.

Confidence Score: 5/5

The PR appears safe to merge, with no concrete behavioral, build, or security regression identified.

The only production change narrows an unused internal export, while the test changes retain coverage at the public seams and do not alter runtime behavior.

Important Files Changed

Filename	Overview
src/local-agent-availability.ts	Makes the single-provider availability checker private while retaining the snapshot API used by production callers.
src/local-agent-availability.test.ts	Replaces environment-dependent helper tests with a deterministic assertion through the snapshot API.
src/local-agent-acp.test.ts	Removes runtime-key implementation assertions while retaining ACP command and sandbox mapping coverage.
src/local-agent-codex.test.ts	Removes resolver-cache and runtime-key representation assertions while preserving Codex behavior tests.
src/local-agent-presentation.test.ts	Keeps structured presentation coverage while dropping exact formatter-string assertions.
src/ui/card-types.test.ts	Tests shared expandability behavior directly; initial expansion currently delegates to that same behavior.

_{Reviews (1): Last reviewed commit: "test: keep agent formatting at CLI seam" | Re-trigger Greptile}