fix: discard subprocess stdout/stderr to prevent LSP stream corruption

[diofeher]

Summary

• TerragruntOptions.Writer defaults to os.Stdout, so run_cmd subprocess output (e.g. ssh-keygen fingerprints) leaks into the LSP protocol stream
• This corrupts the Content-Length headers, causing the language server client to crash with: Header must provide a Content-Length property
• Fix: set opts.Writer and opts.ErrWriter to io.Discard so subprocess output is silently discarded

Fixes #88

The main root cause of the issue was actually fixed by #92, but when trying the configuration from the issue above, it generated a new error on the extension:

❯ Client Terragrunt Language Server: connection to server is erroring. Header
must provide a Content-Length property. {"256
md5":"b3:37:06:8d:56:9b:79:14:07:2b:ab:28:20:92:21:48 ****@gruntwork.io
(ED25519)\n256 MD5:b3:37:06:8d:56:9b:79:14:07:2b:ab:28:20:92:21:48
****@gruntwork.io (ED25519)\nContent-Length: 190"} Shutting down server.

Test plan

• go build ./... compiles cleanly
• go test ./... all tests pass
• Open a .hcl file containing run_cmd("ssh-keygen", ...) in VSCode and verify the language server no longer crashes

[coderabbitai]

Walkthrough

The ParseTerragruntBuffer function in internal/tg/parse.go now explicitly discards both standard output and error output during Terragrunt parsing by assigning io.Discard to the writer options, with the io import added accordingly.

Changes

Cohort / File(s)	Summary
Output Handling Configuration internal/tg/parse.go	Sets Terragrunt parser to discard stdout and stderr by configuring opts.Writer and opts.ErrWriter to io.Discard, plus adds the corresponding import.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Poem

🔇 A whisper becomes silence,
Output cast to the void,
Clean logs, cleaner parsing—
Noise? Discarded with style. ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and specifically describes the main change: discarding subprocess output to prevent LSP stream corruption, which directly addresses the core issue fixed in this PR.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/discard-subprocess-stdout

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