fix: assorted completion issues with ~ and vars

[reubeno]

Adds 5 new automated test cases for completion + gets them (mostly) passing:

• Completion under an empty dir
• Completion of a non-existent relative path
• Complete absolute file paths
• Complete file path containing variable (e.g., $HOME)
• Complete file path containing ~

(Note that we needed to leave 2 tests temporarily disabled as they pass only on certain versions of bash-completion. Rather than further complicate the PR; we will address them separately.)

This entailed:

• Propagating exit results from failed command substitution through to assignments that depend on it.
• Targeted compatibility fix for limited set of printf %q cases
• Return 1 from compgen when no completions were generated
• Only filtering completions based on the token-being-completed for action-generated completions, not for those coming from an invoked function
• Stop unconditionally expanding token-being-completed in non-compgen scenarios

[github-actions[bot]]

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
expand_one_string	3.33 μs	3.33 μs	0.00 μs	⚪ Unchanged
instantiate_shell	60.25 μs	60.08 μs	-0.17 μs	⚪ Unchanged
instantiate_shell_with_init_scripts	29617.80 μs	29118.13 μs	-499.66 μs	🟢 -1.69%
parse_bash_completion	2688.75 μs	2692.36 μs	3.61 μs	⚪ Unchanged
parse_sample_script	3.99 μs	3.97 μs	-0.02 μs	⚪ Unchanged
run_echo_builtin_command	91.09 μs	90.45 μs	-0.64 μs	⚪ Unchanged
run_one_builtin_command	110.43 μs	108.29 μs	-2.14 μs	⚪ Unchanged
run_one_external_command	2063.83 μs	1918.44 μs	-145.39 μs	🟢 -7.04%
run_one_external_command_directly	1019.37 μs	1014.30 μs	-5.07 μs	⚪ Unchanged

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
brush-core/src/builtins/complete.rs	🔴 20.78%	🔴 21.85%	🟢 1.07%
brush-core/src/builtins/printf.rs	🟢 91.89%	🟢 91.3%	🔴 -0.59%
brush-core/src/commands.rs	🟢 87.91%	🟢 85.58%	🔴 -2.33%
brush-core/src/completion.rs	🔴 16.63%	🔴 19.22%	🟢 2.59%
brush-core/src/escape.rs	🟢 83.87%	🟢 85.95%	🟢 2.08%
brush-core/src/expansion.rs	🟢 95.64%	🟢 95.99%	🟢 0.35%
brush-core/src/interp.rs	🟢 90.18%	🟢 90.21%	🟢 0.03%
brush-core/src/jobs.rs	🔴 42.42%	🔴 37.23%	🔴 -5.19%
brush-core/src/patterns.rs	🟢 97.14%	🟢 97.85%	🟢 0.71%
brush-core/src/shell.rs	🟢 76.13%	🟢 76.43%	🟢 0.3%
brush-core/src/sys/unix/users.rs	🔴 13.16%	🔴 28.95%	🟢 15.79%
brush-shell/src/events.rs	🔴 36.46%	🔴 38.04%	🟢 1.58%
brush-shell/src/main.rs	🟢 89.61%	🟢 90.26%	🟢 0.65%
Overall Coverage	🟢 71.93%	🟢 72.15%	🟢 0.22%

Minimum allowed coverage is 70%, this run produced 72.15%

[github-actions[bot]]

Test Results

  2 files    9 suites   56s :stopwatch: 452 tests 452 :white_check_mark: 0 :zzz: 0 :x: 895 runs  895 :white_check_mark: 0 :zzz: 0 :x:

Results for commit 91bb798f.

:recycle: This comment has been updated with latest results.