feat(assert): add optional custom label to assert functions (#77)

[SauronBot]

Issue

Closes #77

Background

When an assertion fails, bashunit derives the label from the test function name. In complex tests with multiple assertions on the same subject, the auto-derived label doesn't give enough context about which assertion failed or why.

PHPUnit solves this with an optional message parameter. This PR brings the same to bashunit.

Goal

Allow test authors to pass an optional trailing label to most assert functions. When provided, it replaces the auto-derived test function name in failure output.

Changes

src/assert.sh

• Added bashunit::assert::label() helper: uses custom label if provided, falls back to find_test_function_name
• Updated 14 fixed-arity assert functions to accept an optional last param as the label:
  assert_same, assert_equals, assert_not_equals, assert_not_same, assert_empty, assert_not_empty, assert_less_than, assert_less_or_equal_than, assert_greater_than, assert_greater_or_equal_than, assert_contains_ignore_case, assert_string_not_starts_with, assert_string_matches_format, assert_string_not_matches_format

tests/unit/assert_basic_test.sh

• Added 4 tests verifying the custom label appears in failure output

Not supported (variadic ${@:2} or $3 already taken):
assert_contains, assert_not_contains, assert_matches, assert_not_matches, assert_string_starts_with, assert_string_ends_with, assert_string_not_ends_with, assert_line_count, exit-code assertions

Note: assert_files.sh and assert_folders.sh already support custom labels via their existing optional params — unchanged.

Example

function test_user_details() {
  local user=$(get_user 42)
  assert_same "Alice" "$(echo $user | jq -r .name)" "user name"
  assert_same "42"    "$(echo $user | jq -r .id)"   "user id"
}

Failure output:

✗ Failed:
  Expected 'Alice'
  [user name] but got 'Bob'