fix(es): Reject negative index retention

[viiccwen]

Which problem is this PR solving?

• Resolves [Bug]: Elasticsearch index cleaner accepts a negative retention duration #9122

Description of the changes

• Reject negative NUM_OF_DAYS values before initializing configuration or connecting to Elasticsearch.
• Preserve the existing behavior for zero and positive retention values.
• Add a CLI regression test for negative retention.

How was this change tested?

• make fmt
• make lint
• make test

Checklist

• I have read https://github.com/jaegertracing/jaeger/blob/main/CONTRIBUTING_GUIDELINES.md
• I have signed all commits
• I have added unit tests for the new functionality
• I have run lint and test steps successfully: make lint test

AI Usage in this PR (choose one)

See AI Usage Policy.

• None: No AI tools were used in creating this PR
• Light: AI provided minor assistance (formatting, simple suggestions)
• Moderate: AI helped with code generation or debugging specific parts
• Heavy: AI generated most or all of the code changes

[Copilot]

Pull request overview

This PR fixes a safety bug in the Elasticsearch index cleaner CLI by rejecting negative retention values (NUM_OF_DAYS < 0) early at startup, preventing deletion cutoff dates from moving into the future and potentially selecting too-new indices for deletion.

Changes:

• Add argument validation to reject negative NUM_OF_DAYS values before config initialization and Elasticsearch client creation.
• Add a CLI-style regression test that executes the test binary as a subprocess and asserts the validation error is surfaced.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
cmd/es-index-cleaner/main.go	Adds non-negative validation for NUM_OF_DAYS before config init / ES client creation.
cmd/es-index-cleaner/main_test.go	Adds a subprocess-based regression test asserting negative retention is rejected with the expected error message.

[github-actions]

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