Truncate discovery command log

[anmarchenko]

What

Truncate the test discovery command log value to 300 characters and log it as a single command field.

Why

When test discovery receives many explicit files, the logged argument list can become very long and noisy. Capping the logged command keeps the discovery log readable without changing the command that is actually executed.

E2E testing

Run ddtest plan in a project where the test discovery exclude pattern leaves many explicit test files, then verify the Discovering tests with command log line is capped at 300 characters and discovery still completes.

Validation run:

• go test ./internal/discovery
• make test
• make lint