fix(codecs): accept short-form syslog severity keywords in encoder

[vparfonov]

Summary

The syslog codec's Severity enum only accepts full-form keywords ("critical", "emergency", "informational", etc.) but the VRL functions to_syslog_severity and to_syslog_level use short-form keywords ("crit", "emerg", "info", etc.). When a short-form value reaches the syslog encoder, FromStr parsing fails silently and the severity falls back to the default Informational (6).

This adds strum serialize aliases so the encoder accepts both forms:

• emerg / emergency / panic → Emergency (0)
• crit / critical → Critical (2)
• err / error → Error (3)
• warn / warning → Warning (4)
• info / informational → Informational (6)

Vector configuration

How did you test this PR?

Extended the existing test_parsing_logic unit test in lib/codecs/src/encoding/format/syslog.rs

Change Type

• Bug fix
• New feature
• Dependencies
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

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