Add regression tests for concat strings through println and shell

[ATX24]

The error

Claimed regression shape:

function main() -> string {
  let cmd = "echo " + "hello_world";
  baml.io.println("-> " + cmd);
  baml.sys.shell(cmd, null).stdout.to_string()
}

The reported incorrect behavior was silent empty shell output after the println consumption step:

exit_code: 0
stdout: ""
stderr: ""

The expected shell stdout is non-empty and exactly:

hello_world

Root cause

No current compiler/runtime bug was reproduced in this branch. The gap was missing CI coverage in baml_language/crates/baml_tests/tests/shell.rs for this specific interaction:

• string concatenation produces a command string,
• baml.io.println consumes that same value first,
• baml.sys.shell later receives the command string,
• stdout must remain non-empty and exact.

That left the compiler2 VM/sysop path exercised by baml_tests::baml_test! without a regression guard for this silent-failure shape.

The fix

Added three Unix shell regression tests in baml_language/crates/baml_tests/tests/shell.rs:

1. concat-built command passed through one baml.io.println before baml.sys.shell,
2. the same command passed through two println calls before shell,
3. a multi-step concat from two let bindings with the println result assigned to _ before shell.

Each test asserts stdout is not empty and equals "hello_world\n".

Verification

Ran from baml_language/ unless noted:

cargo fmt
cargo test --package baml_tests shell_concat --test shell

Result:

running 2 tests
... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 12 filtered out

cargo test --package baml_tests --test shell

Result:

running 14 tests
... ok

test result: ok. 14 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

cargo test --package baml_tests --lib

Result:

test result: ok. 1589 passed; 0 failed; 1 ignored; 0 measured; 0 filtered out

cargo nextest run

Result: completed with exit code 0 after running nextest setup scripts for the SDK fixtures.

Note: plain workspace cargo test reached sdk_test_typescript_node and failed because the SDK test docs require cargo nextest run for setup-script-driven fixture installs; the nextest full-suite run passed.

Issue Reference

• This PR fixes/closes #[issue number]

Changes

• Add focused compiler2 regression coverage for concat strings consumed by baml.io.println and then passed to baml.sys.shell.

Testing

• Unit tests added/updated
• Manual testing performed
• Tested in Linux cloud agent environment

PR Checklist

• I have read and followed the contributing guidelines
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

Additional Notes

No legacy engine/ files were modified; all changes are scoped to baml_language/.

  

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[github-actions]

⏭️ Performance benchmarks were skipped

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To run them on this PR, do any of the following, then push a commit (or re-run CI):

• Add RUN_CODSPEED=1 to the PR description, or
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[github-actions]

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