fix(exec): reject empty command before executor setup

[not-matthias]

codspeed exec with no command — and codspeed run with an empty or whitespace-only exec config target — built a BenchmarkTarget::Exec with an empty command. That empty target survived the full orchestrator (memtrack install, eBPF/libc uprobe attach, exec-harness spawn) before exec-harness rejected it with Empty command in stdin input, surfaced at the top level as the cryptic failed to execute memory tracker process: exit status: 1 after a ~2.3s probe detach.

The command is only ever supplied at two origins, so each is now validated up front — before any setup work:

• CLI (codspeed exec): the positional command is now clap-required, so a missing command fails immediately at arg-parse with a usage message.
• Config (codspeed run): build_benchmark_targets bails when a config exec command parses to zero words, naming the offending target.

Fixes COD-3225

[greptile-apps]

Greptile Summary

Adds early validation for missing or blank exec commands before executor setup, with regression tests covering CLI and configuration inputs.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failures remain.

Important Files Changed

Filename	Overview
src/cli/exec/mod.rs	Requires a CLI command and validates every Exec target’s executable before constructing the orchestrator.

_{Reviews (4): Last reviewed commit: "fix(exec): reject whitespace-only execut..." | Re-trigger Greptile}

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 17 untouched benchmarks

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_{Comparing cod-3225-exec-empty-command-fail-fast (c8d44ca) with main (8611106)}