refactor: isolate TUI run execution state

[mohitranka]

Summary

• group run validation channels, abort handles, and pending state in RunExecutionState
• preserve run lifecycle, cancellation, and shell integration behavior
• update focused run and characterization tests for the state boundary

Validation

• cargo fmt --all -- --check
• cargo test -p forge-tui --lib --locked
• cargo clippy -p forge-tui --all-targets --locked -- -D warnings