fix(filebeat): prevent panic in dissect processor with invalid field name

[mauri870]

Proposed commit message

When the dissect processor is instantiated with an invalid field name
in the tokenizer config option, it panics. The code assumes any string
is a valid field name and doesn't validate the regex results.

If the field name cannot be parsed, return an error instead of panicking.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

Disruptive User Impact

Author's Checklist

• [ ]

How to test this PR locally

It panics on main and returns a startup error with this PR:

go run ./x-pack/filebeat -e -c <(cat << 'EOF'
processors:
  - dissect:
      tokenizer: "%{\n}"
      field: "message"
      target_prefix: "dissect"
EOF
)

# Exiting: error initializing processors: invalid field name accessing 'processors.0.dissect.tokenizer'

Related issues

• Redacted, see linked issues.

[github-actions]

🤖 GitHub comments

Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[mauri870]

Friendly ping for review. Thanks!