fix(tracing): truncate serialized job payload data

[Naoray]

Summary

• Truncates long serialized string values (e.g. payload.data.command) in job payloads to 500 characters with a ... [truncated] marker
• Applies truncation recursively after sensitive data redaction, keeping useful payload fields like job, uuid, attempts, maxTries, timeout, tags, and displayName intact
• Adds 3 new tests covering truncation of long strings, preservation of short strings, and preservation of non-string values

Closes #34

Test plan

• Existing test for job context collection still passes
• New test verifies long serialized command strings are truncated to 500 chars + marker
• New test verifies short string values are not truncated
• New test verifies non-string values (integers, null, arrays) are preserved
• Full test suite passes (282 tests, 877 assertions)

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