feat(assert): assert_command_available

[Chemaclass]

Problem

assert_command_not_found exists (src/assert/core.sh) but has no positive counterpart. Asserting that a dependency is installed — the first thing an installer test, a Docker image test or a bootstrap test wants to check — has to go through the helper:

assert_true "bashunit::is_command_available jq"

which reports the failure as a boolean assertion over an opaque string rather than naming the missing command. The check itself already exists as bashunit::is_command_available (src/api/globals.sh); only the assertion wrapper is missing.

Every other assertion family in bashunit ships both directions.

Proposal

assert_command_available jq

• Passes when the command resolves in PATH (builtins and functions included, consistent with bashunit::is_command_available).
• Fails with a message naming the command, in the same voice as assert_command_not_found.
• Implemented on top of bashunit::is_command_available so there is one definition of "available", not two.

Also consider aliasing to match whichever name reads better next to the existing assert_command_not_found — but ship exactly one name, not two.

Acceptance criteria

• assert_command_available bash passes
• assert_command_available definitely_not_a_command fails with the command name in the message
• Behaviour for shell builtins and shell functions matches bashunit::is_command_available and is documented
• A missing argument is a usage error (feat(assert): a missing argument reports a failed assertion, not a usage error #983 precedent)
• Symmetric with assert_command_not_found: the two never both pass or both fail for the same input
• Implemented via bashunit::is_command_available, no second definition of availability

Repo checklist (agent)

• TDD: RED → GREEN → REFACTOR. Use /add-assertion if helpful.
• Bash 3.0+ only: no printf -v, no +=, no declare -A, no [[ ]], no ${var,,}, no &>>.
• Assertions are bare-named (assert_x), helpers are bashunit::-namespaced. Start every assertion with bashunit::assert::should_skip && return 0 and report through bashunit::assertion_failed / bashunit::assertion_passed, matching its neighbours in the same file.
• A missing required argument must go through bashunit::assert::usage_error, not compare against an empty string (feat(assert): a missing argument reports a failed assertion, not a usage error #983).
• Tests: mirror the src layout — tests/unit/assert/<file>_test.sh. There is no assert_fails; test failure output by comparing against print_failed_test, as the existing tests do.
• Docs: add the entry to docs/assertions.md, then regenerate the bashunit doc acceptance snapshot (editing that file breaks it).
• Update the assertion count in README.md ("It ships 73 assertions…" — currently already stale at 74) and add a CHANGELOG.md line under ## Unreleased.
• Gates: make sa, make lint, ./bashunit tests/, ./bashunit --parallel tests/. Never run shfmt -w.
• One issue = one PR.

[fzlzjerry]

I reproduced the missing assertion on the current main branch and traced the existing command-not-found assertion and bashunit::is_command_available helper. I’ll take this with a focused TDD change: add assert_command_available <command> on top of the helper, cover external commands, builtins, shell functions, missing-command diagnostics, missing arguments, and symmetry with assert_command_not_found, then update the assertion docs, generated doc snapshot, completion lists, README count, and changelog. I’ll run the serial and parallel suites plus make sa and make lint before opening a PR.

[Chemaclass]

Shipped in #1033, squash-merged as e379438.