Fix minimal input backend to handle multi-line continuation

[Osso]

When reading from a non-terminal stdin, the minimal input backend now properly handles incomplete commands that span multiple lines (e.g., commands ending with && or ||).

Previously, the backend read a single line and passed it directly to the parser, which failed with "syntax error at end of input" for incomplete commands like echo a && followed by echo b on the next line.

Now the backend:

1. Reads lines in a loop
2. Checks if the accumulated input parses completely
3. If incomplete (ParsingAtEndOfInput), reads more lines
4. Shows continuation prompt when in terminal mode

Added tests for && continuation, || continuation, and complex multi-line patterns with pipes via stdin.

[github-actions]

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
clone_shell_object	16.97 μs	16.95 μs	-0.02 μs	⚪ Unchanged
eval_arithmetic	0.15 μs	0.14 μs	-0.01 μs	⚪ Unchanged
expand_one_string	1.60 μs	1.53 μs	-0.06 μs	⚪ Unchanged
for_loop	22.69 μs	22.30 μs	-0.38 μs	🟢 -1.68%
function_call	2.11 μs	2.10 μs	-0.02 μs	⚪ Unchanged
instantiate_shell	52.42 μs	52.19 μs	-0.23 μs	⚪ Unchanged
instantiate_shell_with_init_scripts	24437.20 μs	24232.98 μs	-204.22 μs	⚪ Unchanged
parse_bash_completion	2002.51 μs	2022.46 μs	19.94 μs	🟠 +1.00%
parse_sample_script	1.94 μs	1.96 μs	0.01 μs	⚪ Unchanged
run_echo_builtin_command	15.08 μs	15.03 μs	-0.05 μs	⚪ Unchanged
tokenize_sample_script	3.37 μs	3.37 μs	-0.00 μs	⚪ Unchanged

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
brush-core/src/expansion.rs	🟢 94.25%	🟢 94.33%	🟢 0.08%
brush-core/src/jobs.rs	🟠 51.61%	🟠 50.23%	🔴 -1.38%
brush-core/src/results.rs	🟢 80.16%	🟢 83.33%	🟢 3.17%
brush-interactive/src/minimal/input_backend.rs	🔴 0%	🟠 59.21%	🟢 59.21%
brush-test-harness/src/execution.rs	🟢 80.75%	🟢 79.14%	🔴 -1.61%
Overall Coverage	🟢 74.66%	🟢 74.72%	🟢 0.06%

Minimum allowed coverage is 70%, this run produced 74.72%

Test Summary: bash-completion test suite

Outcome	Count	Percentage
✅ Pass	1558	73.87
❗️ Error	18	0.85
❌ Fail	179	8.49
⏩ Skip	339	16.07
❎ Expected Fail	13	0.62
✔️ Unexpected Pass	2	0.09
📊 Total	2109	100.00

[reubeno]

Thanks for your contribution, @Osso!

Our minimal input backend is intended to be very, well, minimal 🙂. I'm curious to know if you actually ran into this while using it or if it was more something you identified while reading or analyzing the code? Regardless of which, we're open to some minor changes to it as long as it stays relatively simple.

For the new tests you've added, did you confirm they ran against the minimal backend? I don't see any command-line arguments specifying that. One great way to confirm could be to check they fail without the fix.

I'd also encourage you to look over the tests and see if there are some ways you could reduce the repeated logic across them. The separate cases are mostly doing the same thing but with different inputs + result expectations. Perhaps you could extract out the common logic or convert them into a data/table driven test of some kind?

[Osso]

Yes I have been using a modified version of brush (alias handling, history handling, ...) as my default shell for about about a week and the minimal backend is being used while piping. There no need command line arguments as it is the default behavior: maybe that's the real problem? should it be using a different backend while piping?

If you are not aiming for that usage we can close this merge request.

Yes I can cleanup the tests.

[reubeno]

Ah, interesting. So you're still using the shell interactively but via a pipe? I think understand a bit better. I'm okay proceeding with the change after you clean up the tests.

Very cool to hear you've been able to build something that extends brush! If you're willing to share, it would be great to learn more. Perhaps you could show-and-tell in our Discussions area or on our Discord (if you're open to it)?

[Osso]

Right I didn't show an example. What I am doing is this:

  ```console
  $ printf 'echo hello &&\necho world\n' | /tmp/claude/brush -s
  error: main: syntax error at end of input

  $ printf 'echo hello &&\necho world\n' | /tmp/claude/brush-fixed -s
  hello
  world

  $ printf 'echo hello &&\necho world\n' | bash -s
  hello
  world

[Osso]

Ok I updated the tests first by factoring them then I changed them to call minimal backend directly in the latest commit. Let me know which one your prefer.

[reubeno]

Thanks! I should be able to properly review this weekend. In the meantime I see 2 checks failing on the PR from non-Unix builds. Would you be able to look at them?