Refactor e2e parse result handling to use try/catch

[bobzhang]

Motivation

• Replace try? + match Ok/Err patterns with direct try ... catch to simplify control flow and make parse error handling explicit in e2e test helpers.

Description

• Update run_single_valid_test and run_single_invalid_test in e2e/runner.mbt to wrap @toml.parse(...) with try ... catch while preserving existing mismatch and parse-error messages.
• Update the valid-suite loop in e2e/e2e_test.mbt to use try ... catch around each parse and keep pass/fail counters and failure reporting unchanged.

Testing

• Attempted to run moon fmt, but moon is not available in the environment (moon: command not found), so no automated formatting or moon-based tests were executed.

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Codex Task

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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