A mock or spy on an unusable command name dumps a bash syntax error

[Chemaclass]

Problem

mock and spy build the double with eval "function $command() { … }", so a
name that is not a usable function name produces raw bash internals in the test
output:

function test_mock_with_args_in_name() {
 bashunit::mock "ls -l" echo hi
 assert_same "ok" "ok"
}

✗ Error: Mock with args in name
 eval: line 58: syntax error near unexpected token `-l'
 ./src/doubles/mock.sh: eval: line 58: `function ls -l() { echo hi "$@"; }'
 ./src/doubles/mock.sh: line 63: export: ls -l: not a function

Passing the arguments along with the command (mock "ls -l" rather than
mock ls) is an easy mistake, and the message points at bashunit's internals
rather than at the call that caused it. The run does continue — later tests
still execute — so this is DX, not correctness.

Which names actually break

accepted by bash	rejected
foo-bar, a.b, x+y, a$b, a:b, foo_bar	whitespace, ;, |, &, (, ), ", '

So the check has to be narrow: plenty of odd-looking names are legitimate
command names and legal function names, and rejecting those would break
working suites.

Fix

A shared guard in the doubles module that rejects only what breaks the eval,
with a message naming the argument and the likely cause — the shape
env.sh already uses ("Only accept valid shell identifiers (defends the eval
below)").

[Chemaclass]

Done in #1139 (merged).