feat: add not contains assertion predicate

[captainsafia]

Summary

• add not contains as a first-class assertion predicate for output and file assertions
• wire the new predicate through parser, runtime evaluation, LSP completions/diagnostics/symbols, and docs/examples
• add parser, runtime, LSP, and integration coverage for the new syntax and validation cases

Testing

• cargo test
• cargo run --release -- tests/integration/*.hone
• cargo clippy
• cargo fmt --check

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