Keep missing Cargo test binaries non-fatal

[cbusillo]

Summary

• replace the deprecated, panicking assert_cmd fallback in codex-utils-cargo-bin
• resolve already-built Cargo binaries beside the current test target when available
• return CargoBinError::NotFound when a cross-package binary is absent
• cover missing and positive target-directory fallback paths
• remove the unused assert_cmd dependency and refresh convergence metadata

Why

Focused core/app-server runs do not always receive cross-package CARGO_BIN_EXE_* variables. Optional callers used .ok() / if let Ok(...), but the helper panicked internally before those fallbacks could run. This masked 14 #707-aligned filters before their product assertions.

Validation

• just test -p codex-utils-cargo-bin: 2/2 passed
• bazel test //codex-rs/utils/cargo-bin:all --test_output=errors: Cargo and unit tests passed; Windows cross test skipped on macOS
• all 14 previously masked Restore MultiAgentV2 child configuration and lifecycle contracts #707 filter expressions reached product behavior with no CARGO_BIN_EXE_* errors (25 cases: 9 passed, 16 exposed real Restore MultiAgentV2 child configuration and lifecycle contracts #707 failures)
• just argument-comment-lint: 902 targets passed
• just bazel-lock-update and just bazel-lock-check
• just fix -p codex-utils-cargo-bin
• just fmt
• git diff --check
• strict convergence against 7034270582cd028e0145eff999afc42e337798dd: 473 guarded paths, 0 violations, 6 snapshots reproduced
• final Opus review: clean
• final Gemini 3.1 Pro review: clean
• JetBrains inspection attempted but environment-blocked by project_open_blocked; verdict remains UNKNOWN

Scope

This PR only restores the helper's non-panicking Result contract. The newly visible MultiAgentV2 assertions remain assigned to #707. The original #706 SDK timeout, proxy, FIFO, and guardian clusters remain separate follow-up stages.

Refs #706
Refs #707