feat: log tex-fmt stderr output on non-zero exit code

[JuliusBairaktaris]

This PR addresses issue #4744 by implementing proper error handling for the tex-fmt formatter.

Previously, tex-fmt failures were silent regarding their cause because stderr was ignored. This change ensures that if tex-fmt exits with a non-zero code:

• stderr and stdout are captured.
• The captured output is logged to the "LaTeX Workshop" output channel.
• The user is notified via an error message to check the logs.

Key Changes

• Modified src/lint/latex-formatter/tex-fmt.ts:
  • Added listeners for stderr and stdout.

Testing

• Manual Verification: Verified using a local installation of tex-fmt and forcing a failure by passing an invalid argument in the settings. Confirmed that error details now appear in the output channel.

Related Issue

Closes #4744

[Copilot]

Pull request overview

This PR enhances error handling for the tex-fmt formatter by capturing and logging stderr/stdout output when the formatter fails. Previously, tex-fmt failures were silent, making debugging difficult for users.

Key Changes:

• Added logging of stderr and stdout to the output channel when tex-fmt exits with a non-zero code
• Added explicit return statement to prevent execution of success path after error handling

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