refactor(cli): collapse provider-null check into Providers.require helper

[rlaope]

Summary

Every CLI command that resolves a typed provider repeated the same 5-line block:

GcProvider provider = registry.findGcProvider(pid, sourceOverride);
if (provider == null) {
    System.err.println(messages.get("error.provider.none", pid));
    return;
}

The block returned with exit 0, so missing-provider runs silently looked like success to scripts piping argus output.

This PR introduces a single Providers.require(provider, pid, messages) static helper in the command package. It prints the localized error and throws CommandExitException(1), which ArgusCli already converts to a non-zero exit. 32 call sites across 31 command files shrink from 5 lines to 1, and missing-provider runs now correctly exit 1.

• New file: argus-cli/src/main/java/io/argus/cli/command/Providers.java (~30 LOC, single static helper)
• 31 commands migrated to Providers.require(...)
• Net change: −95 lines from command files (after counting the +30 new helper and +41 unit test, total diff is +103 / −127)
• Unit test: ProvidersTest covers both branches (non-null pass-through, null → throws CommandExitException(1) + writes to stderr)

Out of scope

Sites that don't fit the helper shape are left alone:

• ClassLeak/CompilerQueue/Events/GcRunCommand — check JcmdExecutor.isJcmdAvailable() directly, not a typed provider.
• ProfileCommand's parallel fan-out callable wraps the message into a ProfileResult.error rather than printing/exiting.
• PsCommand passes "ps" instead of a pid.

Behaviour change

CLI commands that previously printed error.provider.none to stderr and exited 0 now exit 1. This is the correct behaviour for shell pipelines and CI gates — silent exit-0 on a missing provider was a latent bug.

Test plan

• :argus-cli:test passes (incl. new ProvidersTest)
• Full ./gradlew build (excluding integrationTest) passes
• Smoke: argus gc 999999 exits 1 with stderr message
• Verify on user side that no downstream script depended on the old exit-0 behaviour