Prioritize Google Batch API exit codes with fallback to .exitcode file

[jorgee]

Fixes #6481

This PR improves exit code handling for Google Batch tasks by prioritizing the exit code from the Google Batch API over the .exitcode file, with graceful fallback when API data is unavailable.

Changes

• Primary exit code source: Extract exit code from Google Batch API task execution events (most recent event)
• Fallback mechanism: Read .exitcode file when API doesn't provide exit code data
• Enhanced error reporting: Special handling for Google Batch infrastructure error codes (>= 50000, e.g., 50001 for Spot VM preemption)
• Comprehensive tests: Added test coverage for API exit codes, fallback scenarios, and 500XX error codes

Rationale

The Google Batch API provides more reliable exit codes than the .exitcode file, especially when tasks fail early (e.g., Fusion mount failures with exit code 175). In these cases, the .exitcode file may not be written, causing Nextflow to report Integer.MAX_VALUE (displayed as -). By checking the API first, we ensure accurate exit code reporting consistent with AWS Batch and Azure Batch executors.

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