Harden string helper arity and str_split result variables

[codeforester]

Context

Two verified string-library issues should be fixed together because both are small lib_str.sh API hardening items.

Findings

1. str_contains, str_starts_with, and str_ends_with do not call assert_arg_count. Missing arguments silently produce misleading predicate results.
2. str_split declares a local array named fields; if the caller also uses fields as the result array name, Bash scoping causes the assignment to hit the function-local array and the caller array remains unchanged.

Failure Scenarios

str_contains "needle-only"  # currently returns success because needle defaults to empty

fields=()
str_split fields "a:b:c" ":"  # caller array remains empty

Scope

• Add explicit arity checks to all string predicate helpers.
• Rename str_split internal locals to collision-resistant private names.
• Add BATS coverage for missing predicate args and result variable names that match old internal names.

Acceptance Criteria

• Predicate helpers fail consistently on wrong arity.
• str_split fields ... updates the caller-owned fields array.
• Existing string helper behavior remains unchanged for valid calls.
• ./tests/validate.sh passes.

Validation

• ./tests/validate.sh